Originally Posted By: gpshumway
Originally Posted By: MCompact
I am wondering what design characteristic(s) of the Subaru engine cause the oil to shear down. I am not doubting anyone here, but ask because I run M1 5W-30 in the DI turbo motor of my MS3 and the oil stays in grade for over 7,500 miles.
Check the oil at 2,000 miles in your MS3, it might be shearing down and thickening back up.
There's nothing about the Subaru boxer that makes it especially hard on oil relative to other high output turbo engines, but in many engines a thick 20wt is not much concern. Subaru EJ engines however, have fairly loose tolerances (and clearances) for the rods & mains. The factory tune is also a bit suspect on 2007+ cars, with a long delay at full throttle or near full throttle before power enrichment mode kicks in. The engine has to deal with a bit of knock especially on the 91 octane E10 fuel common in much of the country. The net effect is lots of EJ turbos fail by spinning the #4 rod bearing. The failures aren't that common overall, but of those that fail a spun bearing is very common. Broken ringlands (fron knock) are also fairly common. Those of us in the Subaru community who've studied the issue recommend a thicker oil as a basic safety margin against spun bearings.
All of this is true for EJ turbo engines from 2002, and especially 2007+, but not necessarily the new FB and FA series engines. It also doesn't apply to the non-turbo engines as they're a lot easier on oil and don't have the tuning issues.
To the OP:
If you read your owner's manual carefully it will recommend up to 10w50 for heavy use and high temperatures. The use of a light 40wt like M1 0w40, RT6 or TDT is a pretty small step up from 5w30. If you still are nervous about stepping up a grade, use a heavy duty 30wt with HTHS viscosity over 3.5 cSt. A common choice is Castrol Syntec European Formula 0w30. I personally am using Mobil 1 ESP Formula 5w30 which has been working well, it's expensive and a bit of an odd choice in many minds. I'll elaborate if you're interested. Redline 5w30 is the other good 5w30 choice, I've used it extensively. Expensive and in the bearings nearly identical to a 5w40 synthetic.
Here's a recent uoa on PUP 5W-30 from a WRX with a short oci and the viscosity dropped to a 20 grade. Also notice how the OP of that thread mentioned his driving was a bit more agressive than the earlier uoa's on the older PU 5W-30, and check out the difference in the results.
http://www.bobistheoilguy.com/forums/ubbthreads.php/topics/3442654
Some good oils have been mentioned in this thread. If you want an xW-30, just go with one that is not Resource Conserving (a Euro oil, an HDEO gas/diesel oil, or race/street oil).
And if you're still running the banjo bolts (if your MY has them), I'd keep the intervals at a maximum of 5-6k miles.
-Dennis
