Originally posted by jsmith
overfilling can cause main seal failures and the info on some of the user manuals was a bit on the high side.
Yes, it can ruin both front and rear main seals. If the crankcase is overfilled, the oil is hit by the crank journals. It "bubbles" (the proper term is cavitation) and the oil/air "mixture" is highly abrasive. Anyone who boats knows the prop damage that can result if it cavitates.
Anyway, the cavitation will destroy the seals in a surprisingly short time. My dealer overfilled the C230 and it wrecked both main seals in no time. That was the last time they were allowed to change my oil, and since doing it myself, no problems.