Some say that switching from conventional oil to synthetic, particularly on high mileage engines, can cause an engine to leak. What is actually happening is that conventional oil creates build-up around seals that would normally be leaking. Synthetic removes this build-up and can begin to seep past seals. So what is actually happening is that synthetic oil is uncovering the leaky seals that already exist in the engine.
===Engine Break-in===
Conventional oils are recommended for approximately the first 1000 miles during engine break-in of a new or rebuilt engine. The ultra slippery nature of synthetic oils can cause the rings to fail to seat properly, or take much longer than they should.