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.
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.
NOTE: Some newer vehicles with electronically-controlled transmissions require Dexron IIe or Dexron III fluid. GM says its new long-life Dexron III fluid can be substituted for Dexron II in older vehicle applications.
CAUTION: Using the wrong type of fluid can affect the way the transmission shifts and feels. Using Type F fluid in an application that calls for Dexron II may make the transmission shift too harshly. Using Dexron II in a transmission that requires Type F may allow the transmission to slip under heavy load, which can accelerate clutch wear.






