Well let's see...when bearing clearances get up over .0029 or so with flimsy blocks and cranks, most everything running power adders, methanol fueled engines where the guy wants ring seal, as low viscosity oils and most synthetic base stocks don't deal rings very well, some European car makers call for it, and I'm there are others I'm just not thinking about.
Not everyone is looking for fuel mileage.
If you read close, you'll note I'm either going to use 5w30 or 0w20. The last time Castrol offered 10w50 in this country was 1982.
Oh yeah, most aluminum Rod engines and most aluminum block engines can use a 50.
