Rear main seal leak

[njsteve]

Crazy question but are you using synthetic oil?

When I had my Z11 (350/300) a few years back, I put over 1000 miles on the engine rebuild with conventional oil. Not a spot of oil anywhere on my garage floor.

After 1000 miles I changed the oil to the same viscosity synthetic (Mobil 1) and within 24 hours the oil drips started. They were seeping from the rear oil galley plugs near the camshaft.

Are you sure it is the rear main? The galley plug leak drips down to the exact spot where a rear main leak would be expected - around the pan joining surfaces. We had to pull the tranny twice and ended up having to epoxy the galley plugs in to prevent the leakage. We then went back to conventional oil.