Castrol GTX 20-50

Not trying to open a new topic on the OP's thread, but, engine oil viscosity is determined by the internals bearing clearance's. Not by what looks good on the store shelve or what the other guy's are using.

Don't forget ambient temperature and application. An 8.5:1, 318 in a snow plow in winter isn’t going to use the same viscosity as a 10:1, 318 A body doing burn outs in 100* weather. Same tolerance/clearance but different application and ambient temp.