Quote:
Originally Posted by 504
Just a few things to note;
* 90% of engine wear occurs at start-up/first 20 minutes of driving or so. Increasing to a 5W30 WILL lead to a much higher viscosity on start-ups. Oil viscosity versus temperature is an exponential curve and as it dips below 40 degrees, viscosity begins to rise very quickly. The difference between a 0w20 and a 5w30 becomes quite significant at 20 Degrees Celcius and under.
* oil pressure builds as RPMs increase. Thicker oil requires more pressure for the same amount of flow and if the cars oil pump reaches maximum pressure at say 4000 rpm with a 40wt oil, you will get no more flow and hence no more extra lubrication as you keep revving to redline. The same car with a 20wt oil may still be able to provide extra flow to the engine at redline because it requires less pressure to get it moving. (this would be my biggest concern in a car designed for 20wt oil!)
* Thinner oils flow better, there is less resistance and hence less power loss and a more responsive engine. The starter motor works for less time meaning the alternator has to work less to charge the battery which has discharged less.
Moving to thicker oils make even less sense when you look at the viscosity differences at operating temperatures, 2 Cst difference between a 20wt and a 30wt is nothing and as you go higher the difference gets less and less. If anyone was concerned with caring for their engine during high load situations, oil cooler with a 0W20 would be ideal.
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Good argument but there is very very little history with any Subaru boxer running an oil which such a low HTHS. A lot of Subaru owners move up to Rotella and reported fewer spun rod bearings. Of course the FA20 is a new design. But the fact that the service manual posted above lists 5w-30 makes one wonder if the new engine deign is really optimal with the higher HTHS of a 30 weight. I wish I had the answer. A 0w-30 may smart.
Then again the latest ow20 sustina is the best UOA we have seen ( see thread below). It may be the previous success with Rotella in Subaru's was due to higher protection from ZDDP as Sustina is supposed to have advanced additives that go beyond what ZDDP does as far as protection. It interesting to note that the Sustina UOA is much better than any of the Motul 300V UOA's which are full of anti-wear protection.
I'm changing my oil to 5Qt Mazda 0w20 High Moly + 0.5Qt Mobil Racing 0w30. The Mobil boosts the ZDDP to about the level of Rotella. But after seeing the Sustina UOA I wish I had spent the extra $'s for the Sustina. Right now its the only proven oil shown to stop copper bearing wear.
Time will tell but given previous history with Subaru boxers oil choice is critical.