Calsol process oils are refined from a select blend of crudes using a multistage hydrogenation process. This severe hydrotreating process offers the unique ability to produce naphthenic process oils that meet currently established performance standards as well as those to be developed in the future. Careful selection and blending of domestic crudes, step by step computer controlled refining and the ultra high pressure catalytic processes develop excellent product performance. The quality assurance practices that Calumet brings to bear on crude oil selection and subsequent refining processes assure consistent uniformity and product performance.