SM oil is not always an indicator of the better quality oil for a particular application. SL sometimes means that a particular oil, such as M1 HM, simply has an older, higher (better) level of anti-wear additives such as ZDDP, which was limited even more for the newer SM oils of certain viscosities.
If an engine is leaking, common sense tells me to stay away from more expensive synthetics and use one of the HM blends, at least until the leak stops. Maxlife blend works great here. It works way better than some of the common stop leak additives, in my experience. I have less experience with the other HM oil brands.
Also, not all valve stem seals are made of Viton.