Question: I have a 2008 Chevy TrailBlazer SS RWD with the 6.0-liter V-8 and 4L70E automatic, with 42,200 miles. The owner's manual says the transmission fluid and filter need to be replaced at 50,000 miles. Someone told me that, if I go somewhere like Firestone so they can perform a trans flush, they don't replace the filter, saying the flush has chemicals to clean it out. If I ask for the filter to be replaced, they say it will be an additional charge, as they have to remove the transmission pan. If I'm getting the fluid flushed, does the filter also need to be replaced? Would a dealership perform both fluid flush and filter change for one price? On a side note, when should I change the rear diff fluid? The owner's manual doesn't specify.

Answer: General Motors, and every other manufacturer I can think of, does not recommend the flushing of the engine or transmission, even though some dealership service departments sell it anyway. (Hard to believe, I know.) At 50,000 miles, the designated service is to drop the transmission pan, replace the filter, reassemble, and fill it up with new fluid. In your case, the "drain and fill" will replace about 5 quarts, and DEXRON-VI is the one and only automatic transmission fluid to be used, according to GM.

The presumed advantage to flushing is it replaces all the fluid in the transmission. The drain and fill on your TrailBlazer replaces about 5 out of 11 quarts. But in my opinion, the risks of flushing outweigh the benefits due to the possibility of contamination caused by the flushing equipment and/or use of improper fluid. Do the manufacturer's recommended service with DEXRON-VI and a new filter, and you'll be fine.

Yes, some manufacturer service schedules, or electronic maintenance reminder systems, do leave stuff out. And never changing any type of fluid is not a good answer. In this case, I suggest going with the intervals penciled in for the transmission and transfer case fluid replacement. Do the rear differential, transmission, and transfer case (if equipped) every 50,000 miles.


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