It's not the o-ring, as stated the 4.3 is based off the SBC, that issue affected the LSX. How many miles are on the replacement engine? If it was mine I would run some MMO or Seafoam in the crankcase to help clean it out first and then try a 40w oil. If I had to guess I'd say that most likely the oil pump is a bit worn. The 4.3 usually gets a bad rep because it's not really a 350, it's a fine engine and will last for a very longtime and typically without too many problems. They really aren't too picky about oil either, any 5w30 or 10w30 would work well in that engine, I don't think the newer ones ever went to 20w.