I've heard many times that switching to synthetic in a high mileage vehicle will clean the deposits from the gaskets and could cause leaks. I don't get that impression from reading this forum. I'm afraid of switching to Mobil 1 in my zx2 with 86,000. I have always had 3000 to 4500 dino oil changes. I had a mustang that I switched to Havoline synthetic at 60,000 back in '94. The rear main started leaking. Am I being paranoid?