I've been using Mobil One 5w30 Synthetic along with GM EOS in both 68 Beaumonts, no issues so far, rear mains have always dripped a bit even on conventional oil.
You two guys are really lucky. For a month or so i was good on my 1974 400 V-8 and then there came the drips from the real main seal. My 1974 400 has never and i mean never dripped any oil what so ever until i put synth in it. I went back to fossil and it has been leak free ever since.
-- Edited by long stroke on Monday 5th of October 2020 01:21:56 PM
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1957 Pontiac Pathfinder Deluxe sedan restored 261 six
1974 Chevrolet Caprice Estate wagon low mileage original 400 V-8