Autoboxes and manual transmissions are two totally different animals.
A manual box has 90w140 gear oil in it. This stuff is incredibly thick and has astronomical lubricating abilities for incredible amounts of time. Also there's practically NO heat production in a manual transmission. The only real friction is in the bearings. And considering there is only one bearing that has to put up with any high RPM's, there's just not enough heat build up to create any viscosity breakdown. Plus the gear oil is so resilient that any heat buildup doesn't bother it anyways.
Now, in an autobox, the oil has three jobs instead of just one. It's a hydraulic fluid, it's a lubricant, plus it's also the cooling system. And since on an auto box the clutch(es) are submersed in said fluid (along with several other friction creating items), there's an astounding amount of heat buildup in there. So much so that your transmission will be up at 100 degrees LONG before your engine has even come close to moving the temp needle off the bottom peg. Hence why there's an oil cooler in the bottom of your radiator that leads back to the tranny. And considering high heat is the number one destroyer of automatic transmissions, good fluid and filters are paramount to the survival of your tranny since the fluid is also the coolant. And since the same fluid is also the hydraulic fluid that makes all the valves and clutches work correctly, a good fluid is necessary for the proper operation of the transmission as well.
Hence why the fluid and filter change period should be around 30-35,000 miles. Most manufacturer's say higher, but they are talking about a complete system flush, not just a fluid and filter change.
When you drop the oil pan off the bottom of an autobox, there will be only about 2 litres of fluid come out. The other 2 litres are still sitting in the torque converter. Hence you're only getting half of the fluid out to do the change. And since almost ALL garages only do a fluid and filter change and not a full pressurised flush, this will drop your change interval to one third.
If you've left it for 100,000 miles on an autobox without a fluid change, prepare yourself for either a new car or a new transmission, whichever is cheaper because your existing one is already dead, it just doesn't know it yet.
My wife's Dodge Neon (which are KNOWN for transmission failures) is on 122,000 miles on the original transmission and still going strong. Why? Because I beat the idea of fluid and filter changes every 30,000 miles into my father's head so often that he carried 2 quarts of fluid, a filter, and a drain pan in the boot at all times.
My Chevrolet truck is going on 230,000 miles on its original transmission. Why? Because the original owner had a rigorous maintenance schedule that included tranny fluid and filter changes every 35,000 miles.
One of my coworkers has a Buick Skylark that the transmission is dying on it after less than 75,000 miles. Why? He's NEVER changed the fluid in it.
