Sprirax EP 90 was a single viscosity gear lube with an extreme pressure additive. The most widely used EP additive for decades has been sulfur phosphorus. It's what causes the nasty smell. As time progressed conventional automotive gear oils as well as engine oils became a multi viscosity lube. EP 90 is now called an EP 80/90 or a multi viscosity product.. If there are any yellow metal parts such as syncros in a trans don't use an EP additive with S/P. It starts a corrosion process on the yellow metals that can't be reversed. Many specialty blenders have their own gear oils, engine oils, and coolants but these cars are vintage and if you are still using the original drive train why not use the current spec products such as an 80/90, 20/50, etc. The specs of most lubes from various blenders have to meet current specs and most are backwards compatible.