At normal operating temperature of around 170 degrees Fahrenheit , non-synthetic automatic transmission fluid will last about 100,000 miles before it begins to oxidize. As the temperature of the fluid rises, the rate of oxidation doubles and the life expectancy of the fluid is reduced by one half with each 20 degree increase in temperature.
In general:
| Temperature | Fluid Life Expectancy, in Miles |
| 175° F | 100,000 |
| 195° F | 50,000 |
| 212° F | 25,000 |
| 235° F | 12,000 |
| 255° F | 6,250 |
| 275° F | 3,000 |
| 295° F | 1,500 |
| 315° F | 750 |
| 335° F | 325 |
| 355° F | 160 |
| 375 ° F | 80 |
| 390° F | 40 |
| 415° F | Less than 1/2 hour |
Obviously, then, transmission oil coolers are a good thing, especially when towing or severe use is encountered.
Thanks to Gary Pavlovich for supplying this information.
June 9, 2005