80w-90 vs 75w90 gear oil for differentials

[StructEngineer]

Is there any good reason the service manual calls for 80w-90 gear oil in the front differential while it calls for 75w-90 in the rear?

 

I'm not thinking there's much difference, but this is my first AWD car. Was just going to use Valvoline Synpower 75w-90 for both front and rear. Should I be ok??

[idosubaru]

you're fine, that minuscule difference means nothing.

 

it would be spec'ed different just due to the front and rear diff not being anything alike. but it doesn't matter here.

[bulwnkl]

In a mineral gear oil, there's a large difference between an 80W90 and a 75W90. A mineral/conventional 75W90 may/will have a quite large quantity of polymer viscosity index improver which will shear over time. The base oil blend is much lighter, so this means your gear oil will get significantly thinner over time. An 80W90 will have a higher viscosity base oil blend and much less polymer VII to shear. So, it'll be more consistent in its viscosity over time. Also, the thicker base oil blend will afford better protection to the gears.

 

Synthetic gear oils do not use nearly as much polymer VII to get to 75W90, and a very high quality synth will not use any VII. I do not know Valvoline's Synpower formulation so I don't know whether it's using a bunch of polymer or not.

[StructEngineer]

bulwnkl, so what do you use in your diffs?

[aircraft engineer]

The difference is that the syn oil starts with a lower initial viscosity (75) which increases as the temperature increases (without looking up the spec I can't tell you what the changeover temps are) to 90. The 80 conventional oil does the same but starts "thicker" (80 compared to 75) It REALLY doesn't make that much difference because they are not under pressure - it's all "splash". Aside from the hypoid gear faces, nothing in there is under that much contact pressure (and the side bearings don't "care" as long as they have some lube). It's EXTREMELY unlikely that you could do any damage to the system by using 75 in place of 90. (Beats me HOW you could hurt it, anyway)

 

PHYSICALLY - the 2 perform about the same except that the syn oil would be better in very low temps (like Alaska in the winter - where the non-syn oil might act like toothpaste, the syn will act like honey)

 

IF it were for an engine, then it matters a lot more because of starting lubrication, but then we might be talking about, say, Phoenix in the summer where the temp doesn't drop below 100F at MIDNIGHT. They tend to go to 10W-30 or 40 in the summer months there even for 5W-30 or 5W-20 engines. I would BET that the oil for their new DIESEL isn't 5W-30 though, just based on what I know about oil.

 

Remember - SUB set up their specs with what they had available from whatever manufacturer and are interested in maintaining the LEAST possible spread of components ("lean" inventory).

 

What they spec is what they sell, what they sell is what they spec. "One size fits 'US' - all of us" ("us" means "Subs' - 1 material in the factory, 1 material in the shops - less chance of errors)

[bulwnkl]

I disagree with you about this, aircraft engineer. Gear oils are an area where there's a very significant difference between 'conventional' and 'synthetic' oils, much more so than engine oils. Gears chew up VIIs quickly, whereas engines generally don't. Also, gears are a place where higher viscosity itself affords a good deal of protection. Thus, the higher viscosity base blend and lack of VIIs to shear make a conventional 80W90 a noticeably better protector than a 75W90.

 

To answer your question, I use a synthetic 75W90 from a place called Renewable Lubricants. It uses bio bases, which are outstanding as gear lube bases.

[bulwnkl]

Here's where I'm coming from with my statements about gear oils and what they're made of. This is for GL-5 gear lubes for mineral, synthetic, and blend:

 

"A. A mineral 75W90 Gear Lube formulation will contain:

 

7% EP additive package

55% 100 neutral mineral oil

19% Viscosity Index Improver

1% Pour-Point Depressant

 

It will have a viscosity of 14.2 cSt and VI of 102.

 

B. A 75W90 Blend

9.5% EP additive package

20% 600 Neutral mineral oil

25% PAO of 4 cSt viscosity

20% PAO of 100 cSt viscosity

2% Pour-Point Depressant

 

It will have a viscosity of 14.45 cSt and VI of 157.

 

C. A 75W90 Synthetic

 

9.5% EP additive package

24% PAO of 8 cSt viscosity

52% PAO of 40 cSt viscosity

No Pour-Point Depressant

 

It will have a viscosity of 15.48 cSt and VI of 140.

 

D. A mineral 80W90

 

5.5% EP additive package

43% of 200 Neutral mineral oil

51% Brightstock

0.5% Pour-Point Depressant

 

It will have a viscosity of 14.2 cSt and VI of 101.

 

 

E. 75W-140 Synthetic

 

9.5% EP additive package

23% PAO of 4 cSt viscosity

51% PAO of 100 cSt viscosity

15% of diisodecyl ester (a diester or may contain a polyol ester)

1.5% Pour-Point Depressant

 

It will have a viscosity of 26.9 cSt and VI of 148.

 

F. 80W-140 Blend

 

6.5% EP additive package

13.5% 600 Neutral mineral oil

41% PAO of 40 cSt viscosity

17% PAO of 100 cSt viscosity

20% of diisodecyl ester (a diester or may contain a polyol ester)

No Pour-Point Depressant

 

It will have a viscosity of 25.2 cSt and VI of 153.

 

G. A 80W140 Synthetic

 

6.5% EP additive package

26% PAO of 40 cSt viscosity

50.5% PAO of 100 cSt viscosity

15% of ditridecyl phthalate ester (a diester or may contain a polyol ester)

2% Pour-Point Depressant

 

It will have a viscosity of 26.3 cSt and VI of 168.

 

H. A Mineral 80W140

 

5.5% EP additive package

64.5% 200 Neutralmineral oil

29% Viscosity Index Improver

1% Pour-Point Depressant

 

It will have a viscosity of 25.9 cSt and VI of 141.

(Data derived from “Synthetic Luibricants and High-Performance Functional Fluids,” Ch. 17, Marcel Dekker, Ed. Ronald Shubkin)."

Quoted from a post by MolaKule at the site linked below.

See more nifty technical digest-type stuff here.

[aircraft engineer]

I haven't seen any "normal" duty car differential hypoids that have the pressure levels that would require anything approaching the EP (extreme pressure) loads of some power devices. In fact, engine oil was the (manual) trans lubricant on the Mini years ago - did a complete oil and trans oil change with the engine oil. the engine sump was the trans case as well. I seem to remember those being "spur" gears in the final drive, though, which is a different animal compared to hypoids.

 

The point being that for your "garden variety Subaru", the 75W or 80W question or the "synthetic" vs "mineral" oil question might make a difference way out in the service lifetime. For "normal" operations it's so far down in the significance factors as to be negligible.

 

Notice (interestingly) that the EP additive percentages for syns are higher than for mineral. I guess I might bend one of the lubricant experts ears tomorrow if I remember. One I know has for sure forgotten more than I remember.

[bulwnkl]

That'd be an interesting perspective. The person I quoted above is a lubricant formulator (and a physicist). It's always nice to see an opinion from another from the same profession for comparison/contrast.

[PAezb]

This all reminded me I have 75W-90 waiting to be put in my 05 OBW. Peaking at the rear diff, it appears the drain and fill plugs are now Hex-Allen, unlike the 1/2" square plugs on my 96. Anyone know what size Allen wrench these take?

[bulwnkl]

No kidding, allens? My '05 Baja's rear is still the 1/2" square. The front is a T-70 torx since I have an A/T. Sorry, I don't know the allen size.

[PAezb]

No kidding, allens? My '05 Baja's rear is still the 1/2" square. The front is a T-70 torx since I have an A/T. Sorry, I don't know the allen size.

 

Yeah, I confirmed the front is still the T70 Torx. Waiting on a dealership callback on the rear diff's drain/refill plugs. Subaru just loves to force me to purchase more tool accessories for the toolbox with each new MY revision.

 

BTW: reminder to all DIY's that you need to purchase new crush washers for the differential plugs, just like the engine oil drain plug.

[bulwnkl]

BTW: reminder to all DIY's that you need to purchase new crush washers for the differential plugs, just like the engine oil drain plug.

 

Criminy, that's different, too. My rear plug is just a pipe thread (no washers; interference fit). A plug with a crush washer is OK by me, but dang! Let's be a little consistent.

[bulwnkl]

Is there any good reason the service manual calls for 80w-90 gear oil in the front differential while it calls for 75w-90 in the rear?

 

I'm not thinking there's much difference, but this is my first AWD car. Was just going to use Valvoline Synpower 75w-90 for both front and rear. Should I be ok??

 

It occurs to me that the answer I intended to give you was only implicit, not explicitly stated. I do not know why the specification difference, but there's a slight possibility that since most of their vehicles are sold with the A/T, and since those bias nearly all the torque to the front wheels under normal circumstances, they believe that the front diff will experience more wear and thus need a little more protection? Probably not, but there is a degree of logic to that concept.

[StructEngineer]

Criminy, that's different, too. My rear plug is just a pipe thread (no washers; interference fit). A plug with a crush washer is OK by me, but dang! Let's be a little consistent.

 

Hey, do you know how to get to the front diff fill hole?? I can barely even get my hand in there....let alone pull out the dipstick. Is there something that must be removed?...airbox? or do you go from underneath??

[bulwnkl]

Hey, do you know how to get to the front diff fill hole?? I can barely even get my hand in there....let alone pull out the dipstick. Is there something that must be removed?...airbox? or do you go from underneath??

 

It'd kinda tricky, isn't it? I've used a couple things. One is a looong funnel, the other is a short funnel with a hose on it. Neither is perfect (mainly because the thick gear lube doesn't flow too quickly), but each has worked pretty well for me. I prefer the looong funnel for the gear oil and the funnel with the hose for the ATF.

[PAezb]

OK, this is kind of funky, but the dealership told me that the newer subies, normal aspirated engines, auto tranny, still have use the square 1/2" hole drain/fill plugs in the rear differentials. The Turbo's automatic tranny and all manuals have the 10mm hex hole in the plugs.

 

Go figure, and so much for standardizing basic components

 

Andy-Jo, if you're reading this post , can you confirm 10mm hex on your 05 diff's?

[bulwnkl]

OK, this is kind of funky, but the dealership told me that the newer subies, normal aspirated engines, auto tranny, still have use the square 1/2" hole drain/fill plugs in the rear differentials. The Turbo's automatic tranny and all manuals have the 10mm hex hole in the plugs.

 

Go figure, and so much for standardizing basic components

 

Andy-Jo, if you're reading this post , can you confirm 10mm hex on your 05 diff's?

 

I'm not Andy Jo, but my turbo A/T Baja ('05) is a square 1/2" plug for both drain and fill. Reports are that all Bajas like this one are LSD in the rear. Could that be part of the difference? LSD vs. open diff?