Well now, looks as if we've hit a few hot buttons with this topic. In my opinion, Mobil has been less than honest about the seal issue, especially for older blends of this oil.
My father used M1 in the family's GMC Suburban and Toyota Corolla when M1 first became big in the late 1970s. (He bought M1's initial hype about 12K oil changes.) At the time these vehicles had low mileage, so seal wear should not have been a big issue. Both vehicles promptly sprang oil leaks, and he ended up using seal-swell additives in both. Later, when I began driving the Toyota, I got to replace the main oil seals. I switched to dino and swore I would never use a synthetic oil again. My '88 Festiva made over 350,000 miles on dino oil, and only after my '97 Escort wagon got over 100,000 miles did I finally switch to a synthetic--Red Line in this case.
But there's more. In about 1986 my father bought a brand new Tecumseh engine to use in a golf cart. He had bought fresh M1 and decided to put it in this engine just to see what would happen. I was there and witnessed it myself. Right there on the kitchen table, the M1 began leaking out an engine seal even as he was still pouring it into the crankcase! The seal was installed properly and tight. He stared in disbelief, stopped what he was doing, drained the M1, installed dino oil--and the seal held. Only then did he start this engine for the first time. Of course, this was another strike in my book against synthetics.
Today's M1 seems to be an excellent oil, and the seal compatibility problems seem to have been licked. But some of us have long memories. . . .