Question:What is transmission oil needed for?
Answer: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.
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.
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).