Here is my understanding in simple terms. Synthetic oils tend to clean up any residual deposits that are left in an engine, they clean better than conventional oils. Sometimes these deposits are masking leaks that will show up with synthetic oil. Synthetic oil does NOT cause or create new leaks, it just exposes preexisting leaks that may have been plugged up with crap. I switched my 85 over to synthetic around 130,000 and it started leaking like a stuck pig
so I changed it back to conventional oil and it stopped leaking after a few weeks. It may not always be the case, but seemingly no harm was done in my case. I change my oil on a regular bases regardless of mileage (last time was under 1,000) and as a result my oil runs clear after 3 to 5 months of use in the truck. I have heard that synthetic blend won't "cause" leaks but is an improvement over conventional. The Toyota dealer told me that blend isn't a synthetic/conventional mix, but rather a super purified conventional oil I put some blend in my wifes car and noticed that the engine ran quieter and smoother, about the same as when I switch my Tundra to synthetic. I am planning to try blend in my 85 on the next change.
