Shell Spirax 90 EP was a name given to a single viscosity gear oil blended when these cars were being built. For many decades Shell has used names of sea shells to name their products hence the name Spirax. This product contained a sulfur-phosphorus extreme pressure additive that creates a microthin sacrificial film on the surface of the gear teeth which results in preventing wear to the gear teeth. Today we have multi viscosity gear oils instead of single viscosities. The conventional multi vis oils also contain the same EP additive. The multiple viscosity lube provides good lubrication at both low and high temps. If an older product specification called for a single viscosity lube there would not be a problem with using an 80/90 gear oil. The multi vis oils also have rust and corrosion inhibitors, foam inhibitor and a pour point depressant which lowers the temp at which the oil is still liquid and seal conditioners. Most 80/90 will still pour at a temp of close to -30F and withstand temps up to around 400 F.