Dear Mr. Hughes,
Let me offer to you a small excursus in area of transmission oils.
API classification subdivides all transmission oils into 6 classes:
API GL-1 - oils for light conditions. They will consist of base oils without additives. Sometimes they contain small amounts of antioxidizing additives, corrosion inhibitors, depresants and antifoam additives. API GL-1 oils design for spiral-bevel, worm gears and manual transmissions without synchronizers of trucks and farming machines.
API GL-2 - oils for moderate conditions. They contain antiwear additives. They design for worm gears of vehicles. The oils recommend to lubrication of transmissions of tractors and farming machines.
API GL-3 - oils for moderate conditions. The oils are containing up to 2.7 % antiwear additives. They design for lubricating bevel and other gears of trucks. Not recommended for hypoid gears.
API GL-4 - oils for operating in different conditions - from light to heavy. They contain 4.0 % of effective antiscuffing additives. The oils are designs for bevel and hypoid gears which have small displacement of axes, for gearboxes of trucks, for units of axles. These oils are recommended for not synchronized gearboxes of US trucks, tractors and buses, for main and other gears of all vehicles. Now these oils are the basic for the synchronized gearboxes, especial in Europe.
API GL-5 - oils for the most loaded gears, working in severe conditions. They contains up to 6.5 % effective antiscuffing additives. The general applications of oils this class - the hypoid gears having significant displacement of axes. They recommended as universal oils to all other units of mechanical transmission (except gearboxes). The oils which have special approval of vehicle manufacturers can to use for synchronized manual gearboxes only. API GL-5 oils can be used for limited slip differentials if they correspond to requirements of specification MIL-L-2105D or ZF TE-ML-05. In this case the designation of class will be another, for example API GL-5+ or API GL-5 LS
API GL-6 - These oils for the most loaded gears working in very heavy conditions (high speeds of sliding and significant shock loadings). They contain up to 10 % high performance antiscuffing additives. They designed for hypoid gears with significant displacement of axes. Now class API GL-6 is not applied any more as it is considered, that class API GL-5 well enough meets the most severe requirements.
As almost all Transmission Oils have S-P compounds (antiwear or antiscuffing additives) they affect on parts of transmission units from yellow metals. This affect is some problem at use the transmission oils in different applications. Often we can see the results of misusing these oils. For solution problem were advise the antiwear and antiscuffing additive from inorganic borate compounds which no affect on yellow metals. The overall level of these additives meet API GL-5 but as these additives not attack yellow metals the performances such oil are meet API GL-1. The use of inorganic borate compounds allows recommend such oils for all applications - from API GL-1 to API GL-5. It looks like Universal Transmission Oil.
Excuse me for long lecture, but I hope it will useful for you.
