Originally Posted By: Miller88
Synthetic oil doesn't cause it to leak; it clears out the stuff that's stopping the engine from leaking ... there's already something missing and it's covered in crud.
If you have a high time engine that was run on conventional oil and has some degree of sludge deposits, switching to a full synthetic, or an oil that has a lot of detergents can make any oil leaks worse. The reason is what they call the false oil seal effect, where sludge is helping a worn or split oil seal to function. In a bad case you might need to use a stop leak additive to reduce any resulting oil leaks.
The problem is a lot worse if you use a flush additive in an engine that has oil leaks.