If you go to the Mobile website and search around, you can find the spec. sheets on both 15/50 and vtwin. The only difference I found is vtwin has a 35 degree higher flash point, and .0004% more zinc and phos., which are anti wear agents that are supposed to be critical to aircooled engines. There may be other improtant differences, but they are not listed. I made my choice on based on what they published.