I believe that switching to
synthetic motor
oil has "caused"
oil leaks in some of my cars. I purchased two high mileage cars (both had 100,000 miles) and I switched to Mobil One in both cars. The valve cover gaskets were made of cork and I believe they had expaned from long term exposure to conventional motor
oil. By switching to
synthetic, the gaskets appeared to shrink which prompted oil leaks at the valve covers.
Another situation in which oil leaks can occur is due to the increased detergents that are present in some oils. It was explained to me that "varnish" and other deposits can form around seals on rotating parts such as crankshafts. High detergent oils can remove some of those deposits which increase the clearance on the seal around the rotating part and therefore promote a leak. This is probably more common in automatic transmissions where fluid may not be changed at recommended intervals.
I have switched to using
Amsoil synthetic trans fluid and engine oil in my newest vehicle and so far I don't have any leaks. I also spoke with the owner of a transmission shop who doesn't sell
Amsoil but uses it exclusively in his cars and boat and loves it.