In general, the cheapest to manufacture and/or the "heavier" viscosity ranges. Refined heavy paraffin oil contains considerable amounts of like-molecular weight waxes. How the wax is dealt with determines whether "slack wax" is recovered for sale as a commodity. The older method (Group I base oils) use solvent refining in a chilled environment to crystalize a significant amount of the heavier waxes out so they can be filtered and recovered commercially. This solvent refined base stock can then be blended and additized to commercial motor oil. Though some wax inevitably remains in the base stock, pour point depressants can keep it under control in most climates in appropriate viscosity grades. The second means is hydrotreating -- and can be used on both solvent refined base stocks or stocks straight from the distillation tower. The feed stock is subjected to high heat and catalytic action in the presence of hydrogen gas under high pressure. The process culminates in the isomerization of waxes and aromatics into desirable heavy paraffin base stocks. (Note: contrary to popular misconception, "paraffin" does not equate to "wax".) Obviously no slack wax is recovered commercially in this process. The duration of the process progressibvely results in Group II, Group II+, Group III, and now, Group III+ base stocks. These Group III+ base stocks meet and, in some cases, surpass the entry level 140 VI of Group IV (PAO) base stocks, which implicitly also defines their resistance to oxidation reactions as a measure of the scarcity of undesirable waxes and aromatics. Though not completely wax and aromatic-free, Group III+ base stocks give Group IVs a run for their money. Group IVs are, by definition, completely wax and aromatic-free since they're designer synthesized (polymerized) to the desired molecular weight from simple monomers through progressive stages.