I know this has been asked over and over again here but I need to be 100% on this one. After two months I finally got the 420 back together with new bottom end and case bearings, piston/rings, etc. I used to run it on Mobil 1 15/50 like most of you do here. Brian over at Moto-X here in Laval, Qc. Canada who did my cases says to keep away from Mobil 1, he calls it 'car oil'. He is very knowlegeable and used to road race and aluded that my choice of oil was the cause for my bottom end failure. But then again all of you here are very knowledgeable also! What do you think? Should I go back to Golden Spectro?