There's a certain combination of engine block/cylinder head that chokes off the oil flow to the rocker arms. The fix is to center-drill the threaded area of the center head bolt on the passenger's side with about a 3/32" hole, then cross-drill the bolt with about a 1/16" hole a little above the threads intersecting the vertical hole. Chevy used to sell a drilled bolt like that which can be identified by a slot across the bolt head that makes it look like a BIG common screwdriver could be used to install it. It was used to solve the oiling problem you're describing if the oil comes up the bolt hole with the center head bolt removed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!