Fairly certain it's a tallow based, organic oil of some sort.
It has the non-foaming characteristics of gear oil without the high viscosity. It allows it to deal with extremely high RPM meshing gears, while producing relatively low levels of heat, and not foaming up. Motor oil would foam up in a supercharger application I'd think, and typical 90W gear oil would really crank up the power required to turn the thing at high RPM, especially in a modded car.
You'd never think the blower oil to be an important aspect, but I could see it being a pretty big factor in a high-hp M90 build, seeing as a perfectly running M90 in a max-effort car is using up 80+ hp just to turn the fucker.
I really would love to know absolutely for sure what the makeup of the stuff is, but I doubt you could weasel much info out of Eaton.