20w50 is too thick for a rebuilt engine w/500 miles or less. Try a 10w30 or a straight 30w. Brad Penn is a good break in oil that is already formulated w/enough ZDDP to break in a flat tappet cam/lifter combination.
There's plenty of debate on continuing to run a ZDDP additive after break in, some say yes, some say not needed. You'll find oil generates a lot of discourse LOL
Dave
