The level of extreme-pressure additive - GL-4 for manual gearboxes with yellow-metal synchronisers, GL-5 for hypoid differentials. Neither is better.

GL-5 contains roughly twice the EP additive of GL-4, because hypoid gears slide against each other far more severely than gearbox gears do.

That additive chemistry attacks copper alloys, and many manual gearbox synchronisers are brass or bronze.

GL-5 in a GL-4 gearbox can corrode the synchronisers, giving crunching changes and eventual failure.

GL-4 in a hypoid axle leaves the crown wheel and pinion under-protected, and they wear rapidly.

Some oils are formulated to meet both with yellow-metal-safe chemistry, and where the manufacturer approves one it removes the risk of confusion in a mixed fleet.

Because EP additives react with metal surfaces to protect them, and that same reaction corrodes brass and bronze components.

EP additives form a sacrificial film by reacting chemically with the surface under heat and pressure.

On steel gear teeth that is exactly what is wanted; on copper alloys it is corrosion.

Synchroniser rings, thrust washers and bushes in many gearboxes are brass or bronze.

Corroded synchronisers give a gearbox that crunches into gear, especially when cold, and the damage is not reversible by changing the oil.

Look for an explicit statement of yellow-metal compatibility rather than inferring it from the GL rating alone.

It does not - gear oils use their own viscosity scale, and the numbers are not comparable with engine oil grades.

The two scales were developed separately and the numbers overlap in a way that is actively misleading.

A gear oil grade that looks much heavier than an engine oil grade may have similar actual viscosity.

Never substitute one for the other on the basis of the numbers appearing similar.

Within gear oils, viscosity affects shift quality directly: too thick gives notchy cold changes, too thin loses protection when hot.

Multigrade gear oils exist and are the usual choice where a vehicle sees a wide temperature range.

A friction modifier - without it the clutch packs grab and release rather than slipping smoothly, which produces audible chatter on turns.

A clutch-type limited-slip differential works by controlled slipping between friction plates.

The oil must allow that slip to be smooth rather than stick-slip.

Without a friction modifier the plates grab and release repeatedly through a turn, producing a distinctive shuddering chatter.

Some oils include the modifier and are labelled as limited-slip suitable; otherwise an additive is added separately in a specified quantity.

The chatter is diagnostic: an axle that has started chattering on turns after an oil change almost certainly has the wrong oil or a missing modifier.

On the manufacturer's schedule, shortened substantially for severe duty - towing, high loads, wading and heavy off-road work.

Gear oil intervals are long compared with engine oil, and on some vehicles it is described as fill-for-life.

Severe duty changes that: sustained high loads generate heat that oxidises the oil, and wading admits water.

A hot axle entering cold water draws water in through the breather, which is why oil is checked after deep wading regardless of schedule.

The drained oil is informative - metallic glitter indicates gear or bearing wear, and a milky appearance means water.

Check the breather at the same time, since a blocked one pressurises the housing and pushes oil past the seals.

Usually yes where the manufacturer permits it - synthetics tolerate heat better and flow better when cold, but check compatibility and expect possible seal effects on high-mileage units.

Synthetic gear oils resist oxidation at high temperature and maintain viscosity better across a wide range.

Cold flow is noticeably better, which improves cold shift quality on a manual gearbox.

The oil must still meet the required GL rating and yellow-metal compatibility - synthetic says nothing about either.

On high-mileage units, switching to synthetic occasionally reveals a marginal seal, because the better flow finds paths the thicker mineral oil did not.

The extra cost is easily justified on a hard-worked axle or a vehicle that tows; it is less compelling on a lightly used one.

Through a level plug on the side of the casing - the oil should be at the bottom of the plug hole with the vehicle level.

Most manual gearboxes and axles have a filler plug that doubles as the level indicator.

With the vehicle level, oil should just reach the bottom of that hole; if it runs out, it is overfull, and if nothing appears, it needs topping up.

The vehicle must be level, not on a slope or on one ramp, or the reading is meaningless.

Overfilling causes churning, aeration and leaks past the seals, so more is not better.

Remove the filler plug before the drain plug - if the filler will not undo, at least the unit still has oil in it.