Manual gearbox & axle oils

GL-4 and GL-5 are not quality levels

This is the most expensive confusion in the category: GL-5 looks "better" because the number is higher, but the difference is how much sulphur-phosphorus EP additive it carries, not how good the oil is. A hypoid axle gear slides hard between flanks under enormous local loads and needs that additive. A manual gearbox does not need it and, worse, cannot tolerate it: copper-alloy synchronisers are chemically attacked, and the shift goes stiff within a few thousand kilometres. Oils formulated to pass both do exist, but they say so on the datasheet — it is never to be assumed.

Specifications and grades in common use

Manual gearboxes align to API GL-4, typically 75W-80 or 80W-90, or to the manufacturer's own specification — MB 235.x, ZF TE-ML 02 or Volvo 97307 on commercial vehicles. Drive axles call for API GL-5, usually a synthetic 75W-90 where extended drain is required or a mineral 80W-90 on conventional applications; where fleet durability is the point, the reference is API MT-1 and SAE J2360, which add thermal stability and seal compatibility requirements on top of GL-5. Limited-slip differentials additionally need an LS additive, supplied either in the oil or separately. On machines with oil-immersed brakes the axle oil also has to carry the right friction characteristics, or the brakes squeal and judder.

Mistakes that cost a gearbox or an axle

Nearly all of them come from treating one family as another:

A GL-5 axle oil poured into a manual gearbox that calls for GL-4 — the EP additives attack the copper-alloy synchronisers and the shift goes stiff within a few thousand kilometres.
An ATF put into a hypoid axle: it carries no EP package and has nothing with which to protect the teeth, and the result is heard before it is seen.
The LS additive left out on a limited-slip differential; it sounds like a growl on tight turns and ends with worn friction discs.
A viscosity grade chosen "thicker to be safe" on a modern extended-drain gearbox — it raises churning losses and, when cold, delays oil reaching the upper bearings.
A drain interval carried over from engine oil. A transmission has its own interval, far longer, but also its own load and temperature conditions.