About three years ago, I sprang the $28 for the IPD in-line tranny filter. This little device fits into the cooler return line (an easy cut, then inserted with included rubber hoses and clamps) and filters the fluid going back into the pan.

This particular 1990 745 B230F has seen very severe service during its entire lifetime: short trips around town, frequent idling, cold weather, etc. So I have rigorously used synthetics (Mobil 1 in the engine and Castrol Syntec Dexron in the tranny). I last flushed it around 80k miles, installed the filter at 83k miles, and just flushed all the fluid through the lines and changed the filter at 120k.

The fluid was in great shape: still pinkish, with only a hint of dirt color. No residue or sludge came out of the pan. Once I got all the fluid out, I replaced it with Mobil 1 Syn ATF. Then I pulled the old inline filter and cut it open.

This is a quality unit, by the way: relief spring, magnet, high-quality paper, steel case inside. Very impressive and would put a Fram to shame.

Results: the magnet (about 1cm X 4cm) was covered with a light veneer of ferrite wear particles. It was still magnetic, which is remarkable considering the heat in the fluid. The paper was brownish and contained more wear particles. The filter still flowed after nearly 40k miles. The manufacturer recommends replacement after 60k miles, less on the first filter.

Based on these results, I am sold and will put them in all my cars if I can get them to fit. The turbos are a little tight. If you do this, use a Breeze liner or Euro (non-slotted) hose clamp on the hose inserts: they don't tend to cut the hoses and seal better. Also, Wix sells a similar filter, if you can't get it from IPD.