how often do you change it in an 02J?I have found that probably half or more 02A/02J gearboxes I've come across from non-regulars that come in here are ~1/2 L low. Which is VERY low considering the 2.0L capacity. They can seep slowly out over a long period of time and miles, and then usually 5th gear goes.
Cannot speak to the OP's situation, but if you don't KNOW it is full, at least check. Keeping them full is probably more important than changing the fluid once in a while, but obviously if it does get changed it is less likely to go low.
I had water intrusion through the shift tower seal getting wobbled out due to a bad shift tower bushing. I would check that. If the flange seals are a leaking water in they would let fluid out.I had a problem with water intrusion on one of my transmissions. Probably need to replace the flange seals. But I think I was doing changes about every 10-20K, and at one point, flushed it with some ATF, with the car running at the equiv of 40mph on jackstands for about 10 minutes. Drained it out overnight, and put Smurf's blood in the next day. Haven't checked it since, but it's running pretty good.
Otherwise, I usually go about 80-100K between gear oil changes. I think it's a good bet to do it with each timing belt. I know on most of the junkyard 02J's I've installed in my cars, NOBODY'S EVER changed the gear oil. Surprise, surprise.