Still, and yet, Subaru recommends 5w30 on their turbo engines. Why do you think that is? I mean really, why?! I don't know. Is it because 0w30 is hard to find or is more expensive?

Well, to be frank, adding a turbo does not change the clearances in the block or other things they may have fine tuned with our unique high compression fa20 block. Could really just be because Toyota had a say and wanted better emissions, it's likely something far less worth our speculation than we could imagine. As I had said, the FA20 was far different than the EJ motors and maybe they just decided to try a light oil

The FA20DIT in the Forester XT and WRX and the EJ in the STI run 5w30. The newer (?) fb20dit (?) runs 5w30. Every NA engine in their current lineup like the Crosstrek, Imprezza, etc including the BRZ run 0w20.

The increased torque negatively impacts the design oil film thickness.

all turbo subarus run 5w30 all NA subarus run 0w20

Huh. Yeah I don't pay attention to Subarus, wasn't aware of the trend. Guess you've come to your own conclusion then :)

I've read (but I can't remember where) that oils tailored to perform better at temperature extremes break down faster. ie That a 0W-50 oil fill breaks down to be something more like 10W-30 pretty quickly due to long-chain hydrocarbons shearing.

So I wonder if Subaru's thinking is along these lines - that a 5W-30 has greater longevity than a 0W-30?

This graph is quite interesting.

One thing I see is that in addition to the pressures at high RPM/high temp being too low (under the target of 10psi/1K RPM) the pressures at lower RPMs with lower oil temps seem to be excessively high.

From what I've read, my guess is that dropping to a 0W-40 oil (that has a comparable 100 deg C viscosity) will drop the low temp data points to a more acceptable level.


I'm also a bit shocked that pressures were recorded in the 75-90 psi range. I figured the stock oiling system would have a bypass valve that limited excess pressures, but perhaps it doesn't?


In reality, these excess low temp pressures (on their own) would likely only be a concern on a street-driven or dual-use car. And even that could be mitigated by letting the car thoroughly warm up before driving.

HOWEVER, changing oil weight to drop these pressures could also work in concert with a new gear/oil pump drive designed to provide greater oil flow, and therefore oil pressure.

Hmmmm.

The previous data was on a track with a min oil temp of 90C or so.

This data is some daily driving. The scales of the pressure and colour have changed as oil temps here are a minimum of 17C. Lots more pressure. I wouldn't worry about too much pressure. Seeing 200psi on a cold morning isn't uncommon.

I'm having an engine built now and they'll modify the oil pressure relief valve to hold more pressure before it opens.

Do you mean modify it to have it hold more pressure or less? I do not think the relief valve opening is limiting your pressure on track.

My thinking is that having the relief valve open SOONER would allow you to run -50 or -60 weight oil, boosting high-temp high-RPM oil pressures while limiting pressures at the colder temps. It would essentially help "flatten" the pressure vs RPM curve.


In the reading I've done the last few days I did come across articles that talked about excess oil pressure 'washing' out the bearing surface.

Shimming the relief valve makes the spring stiffer, keeps the relief valve closed longer. Holds more pressure. You can see how the pressure raises nicely with RPM and then flattens out. Stiffening this spring should move that transition to a higher perssure/rpm point.

The 10psi per 1000rpm is a rule of thumb for minimum pressure, not a target. High pressure is not a problem.

At cold temps those thick oils don't flow and allow engine wear. Even with a lower pressure on the relief valve. You want the thinnest oil you can that holds sufficient pressure.

My engine will be finished in two days and back in the car within a week. I guess we'll know for sure then.

Looking forward to seeing the results !