They can be any group of petroleum distillates (I - III) so you have to look beyond CAS numbers. The petroleum distillates CAS numbers merely define the viscosity range, not the purity quality of the lube stock. If you're referencing an MSDS, refer to the toxicology section, specifically, carcinogenicity, if it's listed. If the base oils are specifically listed as non-carcinogenic, that fact would suggest they're extremely low in poly-aromatic-hydrocarbons (great solvents, but crappy lubes because they're subject to oxidation and, furthermore, promote chain oxidative reactions with the disirable lube molecules). While aromatics can be reduced in overall volume (>10% aromatics) in Group I's through countercurrent contact with another solvent, hydro-processing to Group II or Group III status is more effective (<10% aromatics - often in single digit rates). Lube formulators may be reticent to divulge proprietary information, but you can occasionally gain insights by connecting the dots in what they do divulge. I'm NOT a chemist or tribologist by training, but I'm finally finding some use for the semester of organic chem I had to take in college forty years ago...
