However, I remember reading that the additional additive packs in most HM oils (I'm specifically referring M1 HM, to be clear) protect many other aspects of a higher mileage engine. ZDDP and additional detergents are the primary reason that I'd choose a high quality HM synthetic even if I didn't have a seal or gasket leak.
Also, I'd put a bit more faith in the oil than I would something like a typical ATF (actually Lucas Trans Fix comes to mind) where seals or gaskets are just "swollen", I'd like to think (hope) that lubricant engineers for SOPUS, Exxon, BP, C/P and the like have worked out the chemistry a bit further into more of a "conditioning" that the oil lends to the seals and gaskets.