Here's the official word from Mobil 1's website. Not sure how much of this is the word of God, and how much is Communist propaganda.
"Is Mobil 1 with SuperSyn™ compatible with the seals and gaskets in new and older vehicles?" "Mobil 1 with SuperSyn™ does not cause leaks. It is fully compatible with the elastomeric materials from which automotive seals and gaskets are made. ExxonMobil engineers are wary of conventional oils that tout their use of additional seal-swelling agents. With extended use, these agents can over-soften engine seals, resulting in leaks. More to the point, an oil additive will not rejuvenate worn or damaged seals. The damaged seal may have been caused by a worn rotating metal component in the engine. If an older engine is in good condition and does not have oil leaks, Mobil 1 with SuperSyn™ provides the same advantages as when used in a new engine. ExxonMobil recommends taking measures to repair the leaks, then using Mobil 1 with SuperSyn™. ExxonMobil also always recommends following the automobile manufacturer's manual for the proper oil to use."