WHAT ABOUT THE TRANSMISSION FILTER?
Logic would seem to dictate that if there is a removable filter in the transmission that it should be replaced periodically. In actuality, this is not always the case. When most people think of a transmission filter, they apply the same logic as they do towards an oil filter. The difference between the operational characteristics of a transmission filter and an oil filter are so different that they cannot be compared.
An oil filter is considered to be a ‘full-flow’ filter, meaning that the entire amount of your engine’s motor oil passes through the filter on each pass through the oil pump. The oil filter will trap any particle of about 10-15 microns or larger, which is roughly the thickness of a human hair. Eventually, the filter will become so filled with particulate matter that it can become too restrictive, causing it to go into ‘by-pass’ mode, which simply allows unfiltered oil to circulate through your engine. Obviously then, frequent oil and filter changes are necessary.
In contrast to that,
a typical transmission filter is NOT a full flow filter. Only a small portion of the transmission fluid passes through the filter as it makes its way through the transmission. In fact, most of the fluid flows around the filter and up into the valve body. The idea is that eventually, most of the fluid will find its way into the filter, and any particulate matter that may be ‘swimming’ around in there will be held in the filter.
When the engine is turned off, all of the transmission fluid that is being held up inside the valve body and filter will drain back into the transmission pan. This all has somewhat of a ‘backwashing’ effect on the filter, as
a lot of the debris in the filter will be forced down into the pan. Many manufacturers put a magnet in the bottom of the transmission pan to collect and hold these metallic-filings and debris, to keep them from traveling through the transmission. This design is what allows manufacturers to make claims of anywhere from 50,000 miles for a filter change recommendation, all the way up to 100,000 miles.
The best recommendation for a classic muscle-car owner is to flush the transmission as often as described above, and to change the transmission filter about every 30,000 miles.