Go with a Gm motor. That is the only thing we install at my shop (dealer motors) The come with anything from 3 years 36,000 mile to 5 year 75,000 mile warranties. If the engine has a problem on the other side of the country you can take it to the dealer and the will warranty it (provided there is still one there).

 

Might also try a straight weight 40 or 50 weight oil w/ your favorite additive like Restore, STP, etc.