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Following up on this. The new 5psi fuel pump solved the problem. I could cruise at highway speeds all day!

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We entirely disregard Subaru's recommendations on oil unless the car is under factory warranty. We average about 1-2 oil changes 5 days a week for customers - usually schedule these at 9am to get them out of the way and out the door. THE ONLY TIME we use anything other than 5w40 (or 5w50 for 400+ HP), is on cars that are under warranty obligation to use 5w30 or 5w20. Everything else gets 5w40 Amsoil Euro FS. This is an XL oil with a 5w40 spec (normal XL is 10w40). https://www.amsoil.com/p/sae-5w-40-fs-synthetic-european-motor-oil-efm/ It's rated for 12,000 miles. We run it for 6,000 miles since the WIX filters rate about there and Subaru's shear their oil and dilute it with fuel (especially the turbo models) causing some level of consumption after about 2500 miles and we can't trust most customers to check their oil. I'm quite sure that all our customers combined have 10's of millions of miles on this oil and we have NEVER seen a failure related to oil breakdown, sludge, or really any lubrication related failure other than customers that burn all their oil up or leak it out and cause mechanical damage from the complete lack of oil. I have torn down engines due to failures unrelated to the oil - in one case we pulled down a built engine due to a cracked cylinder liner - plugged injector from tank contamination caused a lean condition under boost and the high cylinder pressure blew out the sleeve. Even under the SEVERE detonation conditions that resulted in a cracked cylinder sleeve we saw ZERO damage to the bearing journals running 5w40 Amsoil. So I'm not inclined to change my process. And I have reviewed the specs of the new engines vs. the older engines with respect to bearing sizes and clearances and oil pump specifications, etc - there is no difference between the older engines that specified 10w30 and the newer engines that specify 5w20. Subaru changes the spec based on meeting emissions and economy guidelines, and changes the recommendations for different markets. They have in the past offered 40 and 50 weight racing oils with their branding - in other markets. Viscosity is chosen based on engine LOAD. Higher load requires thicker oil so it doesn't squirt out of the bearings under high cylinder pressure. Take note that I own and manage a performance shop - as such my customers are largely composed of "Enthusiasts" who often drive their cars at the hard. I myself drive all my cars hard and have blown up my share of engines personally. Hard driving equals high load. And thus a step up in viscosity is warranted. GD