I've been running Pennzoil synthetic 10W-30 in my 318 "P"-code in my M-body for years now, and a peculiar problem has cropped up causing excessive oil consumptionl. At around 90K, I replaced both intake and exhaust valve seals with Fel-Pro kit SS70014, which is their "premium" polyacrylate valve seal kit for just about every LA engine out there, and all was well...maybe a half quart consumption in 3K miles and it ran as new.
Suddenly, it started gulping oil at around a quart in 1K, the plugs were becoming oil fouled and everything started going to crap. After some investigation, I popped the valve covers and lo and behold, BOTH intake and exhaust seals were swollen to the point of not even working at all, and the exhaust umbrellas had loosened up and wound up right underneath the retainers! I replaced them all again, and went back to near-zero oil consumption, but it happened again! Bendix is useless for consumer complaints, but I think it's the synthetic oil swelling up these polyacrylate seals.
Anyway, I rummaged through my rollaround and found an old set of Chrysler umbrellas for the exhausts that appeared to be Viton, and stuck those on the exhaust while putting new polyacrylate on the intakes...the problem with the exhaust seals went away, but the intakes are swelling up again!
Anyone have a clue as to WTF's going on here? Surely, if these Fel-Pro seals are having trouble with synthetic oil, SOMEONE must know there's a big problem! I know one thing...I'm going to have to go to reuseable rocker cover gaskets soon!