A gearbox contains a great many moving parts. Some of them are submerged in oil and some of them are splashed — that is, the oil is carried to them by other moving parts.

The lubrication is designed to prevent metal-to-metal contact, between teeth on gears, for example. The type of oil used depends partly on the type of gears the manufacturer has designed into the gearbox.

As in the engine, the oil must be able to withstand high temperatures. `Extreme pressure' additives in some gearbox oils act as solid lubricants on gear teeth when temperature rises above the safe limit of ordinary oils.

The gearbox oil level should not drop noticeably between routine oil changes about every 30,000 miles (50,000 km). If you have to add a lot to correct the level, check carefully for leaks (See ).

Stand the car on level ground before you check the gearbox oil level. On most cars the filler plug on the side of the gearbox acts as the oil-level indicator, but a few cars have a gearbox dipstick.

Marks on the gearbox dipstick show the recommended upper and lower limits for the oil level. Pull out the dipstick and wipe it on a clean rag. Replace it, then remove it again to read the oil level.

If it is too low, top up to the upper limit with the correct grade of oil — as specified in the car handbook.

Some cars — the Mini and Metro for example — have a common oil supply to engine, gearbox and final drive. The oil level for the whole system is checked with the engine dipstick.

Keep the oil up to the upper limit shown on the dipstick, but be careful not to top up beyond this mark.

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