Like many myths, this one is based in fact. Early synthetics were made of esters, which were harder on seals, especially those made of neoprene. However, synthetic oils have come a long way since the early 1970s, and they're much nicer to delicate seals. But while synthetic oil won't create a leak, it will find one. Its streamlined molecular structure has no mercy for cracked or otherwise marginal seals. The oil and its additives may even clean deposits from the engine, which is good -- unless those deposits are acting like spackle on questionable seals.
Related to this is the myth that if you started with mineral oil in your car, you can't switch to synthetic oil. As long as your engine's seals are in decent shape, you can switch back and forth to your heart's content. You can mix and match, you can use blended synthetic and mineral oil or you can use mineral oil for 3,000 miles (4,828 kilometers) and synthetic oil for the next 5,000 miles (8,047 kilometers). You could even fill your reservoir with five different oils from five different manufacturers, and as long as they're the same weight, it will do your engine no harm, according to David Canitz, technical services manager at Royal Purple.
So let's consider those popular myths busted and move onto the realities of switching to synthetic oil.