Had been running Amsoil synthetic in SnowMax, until they raised the price to $20L (or US quart). So screw them, I will run Castrol dino oil that costs me 1/4 the price and does just as well. The only difference that I can figure out is that the synthetic may last a wee bit longer before needing to be changed. To each his own though. RP makes some good stuff so to you can not go wrong.