Thread: Oil Question
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Old 04-07-2013, 02:02 AM   #9
Scooby Newbie
Member#: 188219
Join Date: Aug 2008
Chapter/Region: VIC
Location: Vernon, BC
2007 2.5i Ltd.


Regarding the Rotella 5W40 ...from this thread

williaty has been around quite awhile and actually done a lot of testing on the N/A 2.5 engine.

Originally Posted by williaty View Post
I agree that you shouldn't be running 10w40 for two reasons. First, you want the lowest possible cold viscosity you can get. No matter how warm your climate is, there's no such thing as an oil that's thin enough on startup. You always want to use a 5wXX or 0wXX when it's possible to do so without sacrificing HTHS. Second, 40 weight warm is almost certainly too thick for your engine. Subaru factory engines all seem to come off the line towards the tight end of the tolerance. I've yet to see a modern (GD chassis) NA Subaru without very high oil pressure. You need to build about 10psi per thousand RPM. Most of the ones I've investigated have been making 20-25psi per thousand RPM on 5w30 and even more on 5w40. That's just excessive and undesirably high for a number of reasons.
Originally Posted by williaty View Post
The basic 2.5L engine comes with a 7mm pump capped at 72psi. The AVLS engines come with a 9mm pump, also capped at 72psi. The cars will see 100+psi on startup when the oil is cold but that's just to be expected. Once the oil is hot, they're obviously capped to 72psi by the relief valve. However, most of the cars will see that by 2800-2900RPM on 5w30 oil. This means that at highway speeds in the US, the oil pump is constantly in bypass, which is not a good thing. Ideally, if the pump were sized properly to deliver 10psi per 1,000RPM, the pump wouldn't go into bypass until redline, which is a much better situation for the oil.

There's no direct correlation between kinematic viscosity (weight) and HTHS viscosity. However, in general, heavier oils usually have higher HTHS. Kinematic viscosity has nothing to do with engine wear/protection. HTHS has a HUGE amount to do with engine wear/protection. So when people have tried increasing the oil weight and gotten better results, they'd accidentally stumbled onto an oil with higher HTHS. Modern chemistry, however, has given us oils with lower kinematic viscosities and simultaneously higher HTHS viscosities. These are the oils you want to use.
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