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This is not a scientific article; the main purpose is to give you the general idea regarding different motor oils, and how to wisely chose the proper one for your car's engine; also understanding how they do their job, and more... Don't do this! In this Article: ♪ Understanding Motor Oils ♫ Understanding Classification and Nomenclature ♪♫ How to Wisely chose between the Manufacturers given numbers ♫♫ Differences between Mineral and Synthetic Oils, and types of Synthetics ♫♪♫ and Much More! ______________________________________________________________________________________________________________ Motor Oils, Basically Does all these Things: ► First: Engine Oil stops all the Metal surfaces in your Engine from Grinding together and Shearing themselves from Friction, by Lubricating its internals. ► Second: Engine Oil transferes the Heat away from the Combustion cycle. ► Third: Engine Oil must also be able to Hold in Suspension all the Nasty by-products of combustion, such as Silica (silicon oxide) and Acids, also External contamination, such as Dust. ► Fourth: Engine Oil minimises the Exposure to Oxygen and thus prevents Oxidation at Higher temperatures. It does all of these things under Tremendous Heat and Pressure... Part one: the Proper Viscosity & Thickness (SAE Grade) Recommended for your engine. SAE = Means a Standard of the Society of Automotive Engineers. Remember: The viscosity of a fluid describes its resistance to flow. Heavyweighted fluids such as honey have high viscosity and flow very slowly, while Lightweighted fluids such as water have a low viscosity and flow much faster under the same conditions & temperatures. But in the Motor oil World, Viscosity is somewhat confused with Thickness, and those are two different things with different meanings. ☼ Viscosity depends strongly on temperature and is a major factor in the Flow of the oils. In example, the specification “10W~30” (“W” means “winter”) in a motor oil means the viscosity of the oil at 0º C (32º F) is no more than 3.1 poise, while the viscosity at 100º C (212º F) is no less than 0.1 poise. Naturally, all oils gets thin and flows easily when they are at high temperatures and pressures. (Normal working conditions inside any engine). But modern Oil formulas are designed in Labs (Mixed with special additives), to have certain compensation to that thinning, to keep doing their job properly under high heat and pressure. I will explain how this compensation occurs, below. ☼ Thickness of the oil, is about the oil Barrier it could lay, inbetween moving metallic parts to prevent Shearing. It is the "cushion" that bears the friction between moving metal parts, the greater number means thicker "cushion" but, you should keep in mind that the tolerances between moving metallic parts, not always allows a "Thick" oil to flow properly; that is the reason why certain manufacturers, indicates to use -in example- a 5W~20 (Thin) motor oil, while other manufacturers indicates to use a 20W~50 (Thick) motor oil, on same displacement engines; (Compare older engines with up to date ones). You shall never exceed the numbers given by the car's manufacturer, or the engine internals' tolerances will not allow the oil to flow properly / in time, nor to do its job as it should, causing damages to the engines. Obviously, hot oil will flow better than cold oil, but here... ...you can see how they Behave at 0º C (32º F) ► Multigrade oils have the ability to compensate the natural thinning of their molecules, that normally happens with Heat, so they gets their molecules to grow Thicker when they're exposed to the internal temperatures of a working engine; in that manner, multigrade oils compensates the natural fluid's weight reduction due to the heat exposure; in example: A 10W~40 Oil will behave as a grade "10" oil does at Cold-to-Ambient Temperatures, and will become thicker and behave as a grade "40" oil does, when it it exposed to the heat of the engine at its normal operating temperatures. So those oils with a "W" on their SAE Grade, are "Multigrade" Oils, (W for Winter times) their first number on their SAE grade is the "cool temperatures" number: Thinner oil helps to the engine's oil pump to move the oil faster during cold starts when engine is Cool and thus means less shearing by friction during cold starts. But the Oil's molecules will become thicker with heat, so the same oil will behave as the Second Number, intended for "hot temperatures" on their SAE Grade, when the engine Reaches its Normal operating Temps, compensating in order to have enough protective oil film "Cushion" between moving metal parts and prevent shearing. Want to learn How the Oil "Thickness" protective layer prevents Shearing on your Engine? ... Download the .pdf file document at the Bottom of this Page, you can see tests' results with Photos on it. The oil barrier thickness increases with Heat due to the Polymers that where added to the Oil, those absorb temperatures; also Thinner Oils will increase the Oil Pressure on the engine and that is really needed on Cold Starts, to help the Oil to Reach the Farther engine's places, Faster. How do Multigrade oils get Thicker with heat? The answer was given by Mobil 1: Here: ~► http://www.mobiloil.com/USA-English/MotorOil/Car_Care/AskMobil/Multi_Viscosity_Oils_Long_Molecule_Thicker_Oils.aspx ► Monograde Oils (Single SAE Number) will not vary at all and will behave the same at all the temperatures' range; unless it becomes Very hot, when its Thickness 'Could be' Reduced, it is known as "Oil Break Down" ... (more noticeable on Cheap brands with poor Quality) ... That is the inverse situation and could be Harmful for a Very Hot Operating or for an Overheating Engine: That oil under that conditions could Lost its main Lubricating Properties. I Really advice against the use of Monograde oils on Cars, because they Won't flow properly on Cold Starts, remember: Those parts that worns faster inside the Engine are those parts that doesn't get Oil fast enough with Proper Pressure; so a Multigrade Oil will Reduce both the Time working without Oil on Cold Starts and the low Pressure. Monograde oils could still be good for Power Plants, 18 wheels' trucks, etc... But Not ideal for a Car's engine. Keep the Proper SAE Grade in Mind when you Buy your Next Oil. Be Careful with the Viscosity Grade! The Oil Pumps only creates flow, not Pressure; the Pressure exists due to the Resistance on the engine's internals to such Flow; and the hydrodynamic film strength is what prevents metal to metal Shearing. A thicker oil will increase oil pressure in any engine, but only to a point, due to the Oil Pressure Relief Valve that will simply route oil back to the pan after Reaching certain pressure on the System. So, since Pressure is resistance to flow, a 20W~50 motor Oil will increase pressure, but it means that it does not flow as well through the bearings nor to the top of the motor, causing the Low Pressure problems, let me explain: Considering that a 20W~50 oil is 3 to 4 times thicker cold than a 5W~30, when a Cold engine is Started, there will be Restricted oil Flow when it is needed the Most: the oil relief valve will Cut the flow further by opening at the higher pressure and dumping oil back into the pan. In some engines it will take Longer for the oil to get to the cam shafts, and the Small diameter hole in the middle does not facilitate the Flowing to the high viscosity oil. Since the oil flows from Front to Rear on the cam shafts, it is going to take the last Cylinder valves the Longest to get the oil Flow Needed. The oil squirters spray pattern on the cylinder walls will degrade with a high viscosity oil, especially in cold conditions. While a thinner motor oil keeps the pressure in the upper engine, because the pressure is less than the oil relief valve's top pressure, and the system gets well presurized faster. Remember that the Automotive industry changed their way of thinking regarding Engines. In the past, small displacement engines were designed, keeping in mind that they were built to be used in compact cars; and "Long Lasting" was their main concern, as automotive makers wanted to make a reputation; so Older engines are more "loose" which means that their tolerances -the space inbetween moving metallic parts, and tiny passages- was greater, and could be filled easily with thicker motor oils; that is the reason why you can read in the owners' manual from an average 1.8L engine from the 1980's decade, that 20W~50 was usually the oil recommended to be used. If automotive makers wanted performance and high horsepower output, they had then the six cylinders and eight cylinders' engines with Big Displacement, intended for "Performance" you know... But, the automotive industry of nowadays, is using pretty small displacement engines to move big cars, you can see big SUV's with in-line four cylinder engines that has small displacements, but having high power output, rounding the average Hp of a V6 from the past; and what they did to achieve such goal, is -basically talking- to make their engines "Tight" by reducing Tolerances at their minimum, and use metal alloys that resist such kind of internal pressures; but they requires pretty "Thin" Motor Oils to lubricate properly such engines with tight tolerances; that is the reason why you can see 0W~16 and even 0W~8 motor oils on the autoparts stores, nowadays... Long story short: Despite that a 20W~50 motor oil, provides a better "Cushion" between metallic moving parts, to prevent Shearing; you can Not use such a thick motor oil on an Engine that was not designed to be Lubricated with it; tight tolerances will never allow it to Lubricate and do its job as it should; that is the Reason why you should always respect the Manufacturers' given Numbers in your user's Manual, and if they provides various option, Choose wisely (keeping in mind the environmental / climate temperatures of your area), from the numbers given. Part Two: The Motor Oil's Quality. The American Petroleum institute API Has Two ways to classify Motor Oils: the Gasoline Motor Oils are clasified under the "S" Letter (From "Spark" combustion engine) While Diesel engine oil is clasified under the "C" Letter (From "Compresor" Combustion engine) The API has a program to certify that the Motor oils, meets the strict Performance and quality standards put in place by the OEM. The Service Rating is shown in the API “Service Seal” on the product label, that may look like one of these examples: So, The letter that follows the "S" or the "C" will let you know if the Additives Package (Detergents, Dispersants, Stabilizers, etc... ) is Better or Worse for your application. Any Letter from "A" to "Z" could be next to the "S" or "C" Letter, the more Newer Classification will place a higher Letter there, as Follows: GASOLINE motor oils must read as Follows: SA = Early -older- motor Oils, Very Basic and without any Additive. (then the classification continued with: SB, SC, SD, SE, SF, SG,SH... etc...) SN = Very Recent Classification, includes a Complete Additive Package for Gasoline Engines under the APi Norm. DIESEL motor oils must read as Follows: CK = -or any other Letter instead the "K" like "F", etc.- (Could have a Number 2 or 4 Next to it) The CF or CF-4 are for regular Four Stroke Diesel engines. CF-2 (and other classifications bearing the "-2" symbol) are oils for especial Two Stroke Diesel engines. Beware of those Cheap oils, usually with unknown brand, due to their Lowest quality, in example this one, despite of being "Made in USA" is the worst thing to happen to any engine, besides of running out of oil: ► Important Note: if a DIESEL Motor Oil has, Besides its CK, CF, CG or CH Diesel oil APi classification, any "S" Classification (Such like "SN") , also you can pour them on Gasoline Engines; those kind of oils are commonly known as "Fleet Oils" ... in fact I've been using 15W~40 Fleet Oil from Chevron, the "Delo 400" since many years ago in many EFi cars, with excellent results, because those kind of oils has a very High value of Zinc, which is really needed to prevent wear, especially in older engines, while a Standard "Gasoline Only" motor oil, might comply with the same APi Class, but lacking the needed Zinc additive. But if the Diesel Motor Oil Doesn't have any "S" (such like SL or SH or SM) APi Classification, then Avoid pouring it onto ANY Gasoline engine, those are "Diesel-Only" motor oils. I Kindly Suggest you to Check ANY Motor oil to see if it has the Round Seal from the API ... as the above posted Examples. And the Proper SAE Grade inside that Seal's Circle. If you Pour a "Diesel Only" Oil in a Gasoline Engine, it could be somehow Harmful for the Gasoline Engine, due to the lack of the especific additives for this kind of engine; While the Diesel Oils which also carries the gasolinemotor oil Additives Pack included, (usually known as "Fleet" Oils) are Very Good and Outstanding Oils for Gasoline Engines, they also add "Zinc" to the Additive pack, that normally a "Gasoline-Only" motor oil, lack to have. On the Other Hand: if you Pour just "Gasoline only" Non-diesel motor oil on a Diesel engine, the Oil will be Blackened almost inmediately and will fail for Proper Lubrication and Protection of the Metallic Surfaces, Beside other malfunctions; because the Gasoline oil will absorb the Harmful particles that Diesel Combustion filter to the oil and it couldn't manage those, because Gasoline Motor Oils aren't intended for such contamination. The current Service category Rating for Gasoline engines (since 2012) for cars and light trucks, is “SN” The API SN rating is equivalent to the "GF-5" oil rating by the International Lubricant Standardization and Approval Committee "ILSAC" the SN and GF-5 rated motor oils are backward compatible with previous categories' motor oils, and can be used in any older engines. For Diesel engines, The current category is "CK-4" (updated in 2017), while the older category was “CJ-4” (introduced in 2007 for diesel engines that have exhaust gas recirculation). The previous "CI-4" (2002), "CH-4" (1998) "CG-4" (1995) and "CF-4" (1990)categories, can be used in older four-stroke diesel engines. "CF-2" (1994) is the API classification for two-strokeDiesel engines. Part Three: Synthetic Motor Oils. Synthetic Motor Oil... It's a confusing topic, and there's a lot of Rhetoric, largely Because some Manufacturers and Peddlers of Synthetics have made a lot of inaccurate and self-serving claims over the years. Some, actually a lot, of this Rhetoric is pretty Strident and Opinionated. The problem that Most People encounter when switching a High Mileage car to Synthetic Oils, is due to the Detergents in the synthetic oil. The conventional oil "Gunk" accumulates Around the Gaskets and other Places, often times preventing leaks... The Synthetic oil begins to Remove this "Gunk" and things like Oil Pan Gaskets, Main Seals, and Valve Cover Gaskets, start to Leak oil. Now there are "High Mileage" Syntetic motor oils, so the Leakage problem could be, somehow, controlled ... ... But another thing to Consider is, Based on your Car's year model and type of cam / lifter configuration... a Roller Cam system works fine with Synthetic oil; However, with the removal of Zinc from Engine Oils over the last few years, many Flat Tappet Cams have been unhappy with this change. Most Synthetic oils Doesn't contain Enough Zinc and are Not additive friendly ... ... (that is the reason why I preffer Diesel + Gasoline -fleet- Motor Oils: They do have Zinc, as I stated above). Briefly, there are Two Types of "synthetic" Oils on the market. ► Group IV oils consist of Molecules that are synthesized from simpler chemical compounds. This lets the Chemical Engineers to "tune" the characteristics of a lubricant to exact specifications. These oils are "Fully Syntetic" and flows more Freely at extreme Low temperatures and don't Break Down at very High temperatures; also they generally can be specified one or two grades Lighter than a mineral oil, which consumes less energy and saves Fuel. (Energy Conserving Oils) ► Group III Oils are made from Reprocessed petroleum products normally left over after making Crude oil into Gasoline, Diesel fuel, Heating oil and other products... so they're "Half Syntetic" oils, or Syntetic Blends. They're more modestly priced and have many of the desirable characteristics of the higher-priced Spread. In much of the World outside the USA, Group III-based lubricants are not allowed to be marketed as "synthetic." Don't assume that if is a Synthetic Oil, it is so Good... (Read: very Expensive) ...that you don't need to Change it as often. The base lubricant may be way Better, but the Additive package... (which can be as Much as 25 percent of the Volume of the Product in a Bottle) ...can still Become Exhausted, especially if you drive on Dirty / Muddy \ Dusty enviroments, due to Oil Contamination. Also: Unburned Fuel, Partially Burned Hydrocarbons, Atmospheric Dirt, Metal Wear Particles and Blowby Carbon Particles will Build Up just as fast in a Synthetic-Lubricated engine as in one with petroleum-based oil ... ... The Only Way to Remove all that dirt & contamination is to Drain and Replace the oil. I've always recommended 3000-Mile oil Change intervals in offroading driving conditions, despite if you run Synthetic oil or not, and much more extended mileage for City Drivers. So, with that facts in Mind, To switch to Syntetic motor oils on older engines intended for the Average Driving needs, does Not make any sense; Those syntetic oils are Better for Newer engines... (with Closer tolerances and better PCV systems which keep their oils Cleaner) ...or when the Car is build for Race or Special purposses; but otherwise if you offroad frequently, or drive in dirty enviromants, you'll be Wasting a lot of Money unnecesary, because you'll need to Drain the Oil around each 3000 Miles to keep the Oil with proper Lubrication and the Engine internals safely clean. Part Four: Motor Oil Flush. Prior to change your old oil, if you want to switch from a mineral oil to a synthetic oil, I kindly Suggest to use a Motor Oil Flush Cleaner in high mileage engines, especially if you use your car for Offroading purposses or drive in very dirty enviroments. I use this from Motul on my BumbleBeast's Weberized EA82 engine, because I Drive it very Hard, 40% off-Roading (on weekends) and 60% in City Streets (is my Daily Driver), it keeps the tiny oil passages, especially those on the Hydraulic Lifters, very clean. It is Amazing how many Mud, sludge and Dirt those Cleaners could Remove from the Engine, especially if you has never used one and the engine has high mileage. You only need to pour a Bottle to the Old oil Just before changing it, and let the engine idle for ten Minutes... (Follow the Directions on the Motor Flush Bottle, it might vary on Different Brands)...then Flush the old Oil, change the old oil filter, pour fresh oil and Voilá! But I Kindly Suggest you to Not do such Detergent Flush frequently; if you do this cleansing too often, some seals could suffer a leak due to several reasons which includes: Attack from chemical agents and / or too much Gunk removed. If you Drain your engine's old Oil on a Regular Basis and use good quality oil, there shall not be needed a Chemical Motor Flush, in theory ... but every some years / high mileage isn't bad at all, and could aid to the engine's longevity while keepin' the internals and tiny oil passages / hydraulic lifters more clean. Part Five: Motor Oil Filters. ► Always respect the Part Number given by the Engine's Manufacturer, or use the proper equivalent filter, according to the oil Filter's manufacturer Book of cross references; but Never use an oil filter that is not listed to be used on your Engine, no matter if it has the same thread and pitch, or size; because other important things could be Different, such like the amount of pressure needed to activate their internal pressure relief valves, etc... ► Always chose from a good reputation manufacturer; oil filters made by unknown companies, could Fail, making catastrophic damages to your car's engine. It Never worths to save a penny, risking your Safety and the safety of your Family. Have you thought about that such a simple desision could left you Stranded in the Middle of Nowhere, with a Blown engine on your car? ► If you chose to use a Fully Sinthetic motor Oil that could last much Longer inside your Engine, than a regular -mineral- motor oil; Don't forget to also choose a Long Lasting Oil Filter, for synthetic motor Oils; otherwise, your Filter could fail earlier than when you wanted to change the Oil ... ... I Hope this Writeup will Help you to determine which motor oil is the best for your engine, because Motor oils Shall not be Choosen by Brand nor Marketing, but by the API Classification and SAE Grade considering the Specific Application where they're Needed and the temperatures where they shall work. You must choose between the specs given by the engine's manufacturer for Oil Viscosity, and also chose the Additive Pack that best fit your Engine's age & design, your Kind of Driving and the Climate of the World's area where you Live; so the Smart choice is always to choose the better motor oil to Achieve the proper Balance between flow, needed additive pack and usage, because a very Low Viscosity motor oil will help you to save Gas (Energy Conserving) also will Help in Cold Starts in Cold Climates, but will worn faster the engine's internals on Hot Climates than a High Viscosity motor oil; but if you go too High, the low flow pressure during cold starts on Cold Climates, plus the limited space due to tight tolerances / tiny passages on modern engines, will worn things faster too, and newer API Classifications, such like "SN" does Not carry enough Zinc to be safe on Older engines, such like the Subaru EA series of engines, in such case a "Fleet Oil" with enough Zinc is Highly recommended, es explained above. That is the Reason Why car makers put some different Oil Viscosity Numbers to choose from, on their User's Manual; the Lower Viscosity oils are suitable for Winter times or Cold Climates, while the Higher Viscosity Oils are suitable for Summer Times or Hot Climates. In example, these are Scans of my 1983 Subaru Owner's Manual: Here in this case, you can notice that the Lower Viscosity motor oil (5W~30) is Not recommended for Sustained High Speeds, nor for Turbo Engines; only for Severe Cold Winter times; the reason is that said SAE number is a lower viscosity oil which will not fight enough the shearing on Turbo engines, nor on conventional engines during Hot Climates, or sustained high speeds in this particular EA engines. However, Newer engines with closer tolerances usually "Needs" lower viscosity numbers and thus explain the need of SAE grades such like 0W~20 in example... If you pour higher viscosity number on those Newer engines, the oil will not Flow as intended, nor reach the pressure and areas where is needed, so pouring a High viscosity numbered oil could be Dangerous in certain cases. Always follow the Manufacturer's given numbers, and chose Wisely I Found an important Document which have a Professional Oil Test, done between some Different oil Brands and between Petroleum Based Oils and Synthetic Oils; they compare the Wear from Shearing on a metallic part after the Test, using each Brand of Oil with same procedures for all; it has Pictures, so you can Easily "See" which oil has the Worse (weak) protective cushion / Film Layer and which oil has Better (Stronger) Protective cushion / Film Layer, between metallic moving parts, so you can easily have a better idea. You can Download the Document, a .pdf File with less than 5 MB, Here: ~► http://www.mediafire.com/?lrm2am5vaxk2wob Finally, always follow the Owners manual's instructions, to do Properly the Oil & Filter Change, on your vehicle, Not like this: Don't do this! If you find useful information on my article, Please let me know by hitting the "Like" Button Kind Regards.