Elastomer-seal compatibility may be influenced by the type of base oil present. Paraffinic oils exert a minimal effect while naphthenic oils may cause some seal materials to swell. Mineral Oils Mineral stocks are produced by a number of processes depending on the crude oil being refined. For this reason, the choice of crude is important. The viscosity of a finished stock is determined by the boiling range of its components.
Most refiners settle for three or four stocks from which they blend their range of finished oils.
For solvent extracted HVI oils, VI in the range of 90 to 100 is usual. An alternative refining process, which substitutes deep hydrogen treatment for solvent extraction, can yield VI of over 100. An additional advantage of this approach is that such processes can increase the yield of HVI components from almost any crude. Not all base oils have similar physical or chemical properties or provide equivalent performance. The API has established base oil categories to provide guidance to users when different base oils are used interchangeably.