I think you are oblivious to the Point that Turbs as Making about the situation where Oils are made for different conditions and when those conditions vary a lot, it's difficult to get ONE formulation to cover all Purposes. A Limslip Differential is a case in Point where a Hypoid drive requires a GL5 spec oil and Most Limslips require a GL4. Combined Engine Gearbox and clutch requires a Compromise oil. where if the Parts were Separated as to the OIL used, they could be better lubed with a more specific oil., Most Cars have separated Engine, Trans and differential oil types,. In the Mid 70s engine oils had to be able to cope with Cat. Converters and had to have some friction Modifiers removed. They THEN became unsuitable for gear boxes. Not everyone Knows this and still follows the original Makers recommendations in cars Motorbikes & tractors etc and thereby risks severe damage to gearboxes. and diffs. Nev