I'm going to be switching my '05 WRX to synthetic oil at 10,000 miles. I'm leaning toward M1 5W-40, because it appears to be M1 Delvac, and it will also work in my Toyota pickup. The service manual does not mention 5W-40 as a recommended viscosity. I am having my oil changed at the dealership unti the warrantee expires, simply to avoid any doubt on their part that I'm keeping up my end of the bargain.
Is there any reason why I'd have a warrantee issue by using 5W-40?
Any concerns with the 40 part of 5W-40? I live in Wisconsin, but the car will be garaged overnight with a block heater timed to kick on 2 hours before leaving for work.
I may opt for M1 5W-30, but I've not been a big fan of how it seems to disappear in an engine.