I would change the ATF approximately every 8k miles. Oil should be 3k to 5k miles. Having recently done a replacement of the water pump and thermostat, I have learned a lot about the cooling system in the e34. As was mentioned, that doesnt have to do with the air conditioning, but rather the cooling system of the engine (keeping it from overheating). With this car you really have to be careful with the cooling system. The waterpump and thermostat are most likely the culprit for the problem, with the waterpump having a plastic impeller. But if im not mistaken I think they rectified that problem and put in metal impellars in the 95 e34s. They are quite inexpensive parts, somewhere around 30 bucks each at BMA. If your car is overheating though, you should fix it immediately, because if it persists you can warp your head gasket (a repair that is not very cheap). And yes, visit www.bmwe34.net that site is awesome.