I have the same engine from a 99 s-10 and honestly I've tried 15w-40, 5w-40(Rotella Syn) 10w-30, 5w-30, 5w-20 ... now i've not dun any sort of testing on the oils but the engine runs great on all of those oil grades. Pick one or something like that.
NOTE: Other then the Rotella Syn I used which...