There's no reason for not being able to switch to synthetic. The myth about synthetic causing leak is totally false. If it leaks after switching to synthetic, it's because the seals/gaskets are already worn. The sludge was probably keeping the oil from leaking out, and that's not something you would want to have. If it leaks, fix it the right way with the right tools and parts, not with engine sludge and comtaminants.
Originally posted by Evolution:
If you started with non-synthetic, stay with it. And for non-synthetic, it's not recommended to go above 5W-30.
