It's all based on allergies and viscosity. Some people are allergic to certain oils. Grapeseed oil is the least viscous (thinnest). Just preference really.
Cottonseed is the 'standard' pharmaceutical oil due to its lack of allergens in most people, but some prefer others. It is really just up to the person making the gear.... Hell, I have seen people use canola oil from wal-mart! LOL! I wouldn't recomend it, but they didn't have any problems with it....