Originally Posted By: Loobed
Originally Posted By: arh
If so, likes or dislikes? Thinking about trying it out in my 2002 GM 5.3
I don't suggest you use a 40 weight in the 5.3 motor. I have a 2001 5.3 and tried the 5w-40 shell Rotella-T synthetic. I did not get any more performance of mileage from that oil. The best mileage I got with that engine is with Castrol 0w-30 (German Castrol). I think the engine tolerances are made for 30 weight. Try the Mobil-1 0W-30 Green cap
Unfortunately, the OP didn't give us much info. If he is considering a 40 wt because his engine is worn and consuming lighter oils, then using a 40 would be legitimate.
Otherwise, you're generally correct, except for one thing: you mean his engine
clearances were designed for 30 wt oils.
Tolerance is how much deviation is permitted from designed ideal clearance. Don't feel bad, you ain't the first one here who's made this error, and you surely won't be the last.