I have used the Rotella syn 10/40, thought it a little too thin, my clutch didnt like the V-Twin Mobil 1 20/50. So far for me, the dino 4-stroke M/C Castrol 20/50 has worked the best, and used less oil between oil changes. Ive never used the Motorex 15/50. I started using Valvoline 4-stroke dino oil in an old Ironhead Sportster Im building, but too soon to tell... Just my 2 cents. BTW, I like oil threads, knowing something an be gleaned from them.