Manual Transmissions:
Some older import vehicles (like Volkswagen Beetles) recommended periodic lube changes for their gearboxes, but no modern car or light truck requires it. The reason why is the oil stays relatively clean and runs fairly cool. Unlike the fluid in an automatic transmission that is being constantly churned (which generates heat) and contaminated by particles worn off the clutch plates, the fluid in a manual transmission or transaxle has life pretty easy. So it usually lasts the life of the transmission.

The only reason you might change the fluid would be if you were experiencing hard shifting problems during cold weather. Most older rear-wheel drive transmissions use a heavy gear oil like 75W, 80W or 90W, which can get pretty stiff at subzero temperatures. Changing to a lighter oil may improve shifting.

Automatic Transmissions:
According to the Automatic Transmission Rebuilders Association, 90% of ALL transmission failures are caused by overheating. Most of these can be blamed on worn out fluid that should have been replaced. For optimum protection, change the fluid and filter every 30,000 miles. What kind of automatic transmission fluid should you use in your transmission? The type specified in your owner's manual or printed on the transmission dipstick. For older Ford automatics and certain imports, Type "F" is usually required. Most Fords since the 1980s require "Mercon" fluid, which is Ford's equivalent of Dexron II. For General Motors, Chrysler and other imports, Dexron II is usually specified.