I'm working on a 2003 Monte with a 3800. It suddenly dumped 4 quarts of oil while driving on the freeway. Got it home and in the air, removed RF wheel,inspected oil filter for punctures, refilled with oil,let it idle and revved it for ten minutes, not a drop of oil leaked. Replaced oil filter anyway and installed a new pcv valve for good measure,left the oil fill cap loose to vent,then took it for a drive and about 5 miles later it dumped it'* oil again. I can see that the oil is coming from behind the balancer but am at a loss as to why it only leaks under load and not at idle? Any ideas?

Check your oil pressure sensor. Pull the connector off and see if there is oil in it.