A friend of mine recently used Can. Tire Synth. Blend and has a serious oil leak now. I too had leaks when switching first to Syntec in 1998 and this past spring when trying Havoline 5-40 synthetic.
Remember reading that M-1 uses Esters to control seals and prevent catastrophic leaks as seen in the 70's. Do Group III oils need these as well? Do they have them?