Transmission

Fluid and filter change is recommended by most manufacturers every 36,000 miles. Where there is a lot of differing opinions has to do with a transmission flush.

During a fluid and filter change, only about half of the fluid drains out and gets replaced, but that is enough to replenish the worn-out additives. Replacing the filter removes any harmful metal chips that we don't want circulating with the fluid.

The jury is out on the value of flushes. In my limited experience, people already have a symptom they are trying to solve, and they hope a flush will do that. The procedure can dislodge debris that was sticking somewhere and was causing a symptom to occur, or was preventing a symptom from occurring. If there was already a mechanical problem, such as a dry-rotted and cracked rubber lip seal, or a broken metal part, no flush is going to solve that. If dislodging some debris made the symptoms worse, the flush gets the undeserved blame, but we forget the underlying problem was already there, and that is why we asked for the flush.

Flushes also involve adding a can of highly-concentrated detergent shortly before the flush is started. That can be aggressive enough to pull some loose fiber material off the clutch plates, and that can result in slippage and the need for a rebuild. It is important to understand that material was already loose and prone to crumbling off. The flush just made that happen sooner. Here again, the flush gets the undeserved blame for causing the problem. The problem was already developing. The flush just made it come to its conclusion faster. Had you not had the flush performed, the same problem would have occurred within the next few months or perhaps the next 10,000 to 20,000 miles.

Not many people come in and ask for a transmission flush when they are not having a problem. The flush can not cause a part to break, but some mechanics believe clutch plate material has been breaking off the fiber plates over time, and that material is needed to maintain the plates' friction characteristics. Draining or flushing the fluid is believed to remove those particles and lead to slippage, then the need for a total rebuild. In reality, you definitely do not want that fiber material circulating in the fluid. That will tear up the rubber lip seals in no time, leading to serious slipping problems. Those particles get trapped in the filter, so to say they are needed to maintain the plates' friction properties is nonsense.

My opinion is scheduled filter and fluid changes are needed for normal preventive maintenance, and for that, upgrading to a flush has no value. If you are already having a problem, there is little chance a flush will solve it. The exception might be if excessive varnish has built up in the bores in the valve body, but that is a result of lack of regular fluid changes.

The difference between brands of transmission fluid depend on the advertising you believe. The "best" brand is the one the salesman has on his shelf. There are some different formulations that have specific additives needed by some transmissions. If you have the fluid and filter replaced at the dealership, they will have the correct fluid recommended by the car manufacturer. If you have it replaced at an independent repair shop or transmission specialty shop, they will have what they have had good results with. If you do this yourself, the best is to buy the fluid from the dealer's parts department. If you go to an auto parts store, the people there will usually know what has generated the fewest complaints, or they will have books that list the correct fluid for your car.