What is transmission oil needed for?


Transmission oil is a motor oil specifically manufactured for transmissions and differentials in cars. Compared to engine oil it is of a higher viscosity which ensures good transfer and better protects the gears. This heavy oil can create viscous drag and lead to inefficiencies in operation.
Some modern automatic transmissions do not use such a heavy oil at all but lubricate with the lower viscosity hydraulic fluid, which is available at pressure within the automatic transmission.

Transmission oil is usually associated with a strong sulfur smell.

Replacing (draining/ refilling) transmission fluid?

Many owner's manuals will say it is not necessary to replace it - so that they can sell you an expensive new transmissions after yours burned out.
The old-fashioned recommendation is to replace the transmission fluid every 36,000 miles. Some newer automatic transmissions (after 2002) will be fine for 100,000 miles. On many cars you will find it difficult to drain since they may not have a drain plug and you'll have to use a pump. Or you may only drain 1/3 out of the dipstick tube, thus dilluting the old with the new fluid. This is not really a substitute for a full fluid and filter change, but you can dilute down some of the old fluid.

If you do it yourself - or just the partial replacement - do not overfill the transmission. Too much fluid can cause it to foam, which in turn can lead to erratic shifting, oil starvation and transmission damage. Too much fluid may also force automatic transmission fluid to leak past the transmission seals.

A shop will do a transmission fluid replacement for approximately US $100 (as of summer 2005).