You change your oil every few months. Did you notice BMW specifies changing the ATF once a year? Here are some notes to help you.

I performed this job on my 1986 325's 4HP22 transmission. The job is moderately difficult; I would recommend it to someone who is comfortable doing things like oil changes and brake jobs.

Ingredients:

10mm socket wrench on a short extension. Torque wrench recommended for reassembly.
5mm hex key (allen wrench)
Torx (star) T-27 screwdriver
ATF filter kit
3 qts ATF dexron II or III
drain pan, funnel, newspaper or cardboard, mechanics coveralls or old clothes

The filter kit includes a filter, an o-ring, and a pan gasket.

Procedure:

jack up the front of the car and set jackstands, or drive up on ramps. You need room to maneuver. Put some newspaper or cardboard under the trans because it will drip ATF.

The trans has a small drain plug at the rear of the pan. Set the drain pan under the plug, open the plug with the hex key and drain the fluid. Now you can drop the pan. Notice there is a tube connected to the front of the pan. This is the fill tube, where the dipstick is. It tries to get in the way when you are replacing the pan; but disconnecting it could be more difficult.

Replace the plug, and undo the six bolts that hold the pan clamps. Mine were not very tight. The bolts will come away with the clamps on them, and the pan will drop. The pan gasket should not be glued to anything; it should come right off.

Now everything you see will be dripping ATF. The filter is the flat metal thing towards the front. It is held on by 3 long torx (star-shaped driver) T-27 screws that are screwed in tight. Undo and remove; remove the old filter and O-ring. Get your new filter and O-ring and install with the screws.

Now reinstall the pan with the new pan gasket. The gasket has a U shaped cross section and just sits on top of the pan. It is trapezoidal in shape so make sure it is fitting correctly. Push the pan back in place - here is where the fill tube bothers you. Just try to push it back up through the engine compartment. A helper on the top side may be useful.

Take the six pan clamps and get the bolts just started. That way you can move the pan to the right place before tightening them. Feel all around the pan to make sure the gasket is sitting right, and the pan is centered on the trans; then tighten the pan bolts. Not extremely tight; the spec is 12 ft./lbs.

Now you are ready to fill. Add ATF until the level is correct on the dipstick. Start the car and run for one minute. Check for leaks. Check the fluid level again and add as needed. Now start the car and, with your foot firmly on the brake, move the gear selector from park to 1 and back, pausing briefly in each gear.

Go for a short, slow test drive and check the level again.

Check the fluid level daily for the next few days, remembering the usual rules: check when hot (after a long drive of 15 mins or more,) idling in park, on level ground. Be very cautious not to overfill. Initially, my transmission was a little strange, but it turned out to be overfilled and now the car is shifting much more smoothly.