There been some discussion about which rinse oils for an additive and " does synthetics cause leaks " lately on the forum . Both the API and ACEA and the engine makers have multiple tests for engine seal/oil compatibilty as seen here http://www.ethyl.com/nav/default.asp?sec=products&page=la Just click the Lubes Handbook to view the many . I think along with Redline that seal leaks with synthetic are VI related only these days .Their website discusses this with gear oil as the topic . Also , since the new group II's and III's don't have the inherent solvency the group I basestock had these formulated oils have multi funtional additives that serve as seal conditioners . What I'm getting at is motor seals should last equally overtime with the better dino's but, given these dino's can and do usually run 15-20f hotter in summer the seals maybe would not be in quite as good a condition as the same motor run on synthetic for 10 years as an example only . Overall I think it's a toss these days in the dino vs synthetic other than the heat factor and potentially the modern dino formulations will keep an engine seal in better condition vs dino's of say 6 years ago even w/o the group I's solvency .