So we get quite a few people coming into our shop for service asking us to inspect there vehicle for needed maintenance. Very often I or another technician will recommend a transmission flush based on milage or based on looking at the fluid on the dipstick (when available). I would say about 30% of our customers decline the service because they have been told by someone or there owners manual that the transmission does not need to to be serviced.
Why would the manufacturer tell the customer that they don’t need to change there transmission fluid? The fact is that automatic transmissions may have more gears and use better synthetic fluid which in theory should allow a longer service interval, however the transmission components themselves are still the same (except for CVT) and there are still friction clutches that wear and leave debris in your fluid. Plus the heat from everyday driving degrades the quality of the fluid overtime.
The manufacturer covers your new car anywhere from 3 years 36k miles to 10 years 100k miles. So after that your on your own. The typical life expectancy of a transmission that does not get regular service is 80-100k miles based on my 10+ years of experience. You decide how long you want your transmission to last.
Typical cost of a transmission service is around 200 dollars and you only need to do this IMO every 30-40k miles to keep the trans clean. Typical cost of a replacment transmission $3k+ and that’s for a rebuilt unit.
You see there is this thing in your transmission called a valve body. It has small passageways and precision valves inside with minut clearances. When your trans fluid gets old and contaminated it starts to act as a micro abrasive slowing down if not stopping completely the movement of valves in your valve body that is critical to smooth and reliable operation.