20W-50 in the summer, 10W-40 in the winter but above all else, make sure that the oil has ZDDP!
There are some oils on the market that have ZDDP, others on this forum will know them off the top of their heads. I buy the stuff (ZDDP) from Moss and add it in when I change the oil. That way I KNOW it's in there.
On their death bed, nobody ever said, "Gee I wish I'd spent more time at work!"