I have used Golden Spectro oil since 1988, I tried Mobil 1 motorcyle oil , and the rear main seal started leaking about 8 weeks later. It got so bad I had to change the rear main seal, about 4 weeks later, a large leak developed again, it was suggested that I get the Mobil 1 out of the engine, and see what happens. I drained the Mobil 1, and went back to Golden Spectro, and the leak stopped with the next ride. The rear main seal material is Viton, the General Electric jet engines that are installed on the Boeing 737's that I work on have numerous Viton material seals, and they are problematic and cantankerous as well, so I am staying with Golden Spectro 20w50, as the seals seem to be happy with this oil. Before I moved to a suburb of hell (Phoenix) I used Castrol 20w50 and 10w40, and had no problems. I replace the oil and filter every 3000 miles (4800 km.).