I don't think the blame for
oil leaks with
synthetic lays with the engine builder. Some engines, like FE's, just don't lend themselves well to it's use. Not every application can be justified for synthetics, especially as it concerns break in period. I won't use it in my 650cc bike either, but many do. I remain skeptical of it from a cost/benefit analysis standpoint. The extra long miles between
oil changes is a moot point due to the low miles my Cobra acquires on a yearly basis. I change
oil based on TIME, not miles driven. Same for my Jeep, I don't drive it enough miles to justify the added expense and 'risk' of
synthetic oil.
As to STP leaving a varnish, to that I also remain skeptical in a real world application such as a low mileage vehicle! Again cost/benefit analysis leads me to believe it's a good product, and worth the 'risk'. You would have to GREATLY over use the suggested amount to increase the viscocity rating an appreciable amount, all things should be done in moderation!
People ask me all the time what oil I recommend, by standard answer has always been: Select a product you like and one that is EASY to find most anywhere and STAY with it, then change oil on a regular basis. I think the worst thing you can do is 'mix and match' different oil brands and additives.