I recently switched to Rotella 15w40 dino oil in both my bikes. I've been happy with the way they shift with this oil. I'm tempted to try the synthetic oil but it is 5w40. I don't fully understand the lower number part of a multi viscosity oil. Is it a 40 weight oil that flows like a 5 weight? Would the low 5w part cause any issues or leaks? The bikes are my Vulcan 1500 and Yamaha XT225. I'd also use it in various ATVs.