What kind of ATF fluid do I need for a 1998 540i? :help
:stickoutt

Any Dexron III is fine for the Power Steering system. For the transmission you need to check the sticker on the rear of the transmission for the correct type fluid.

The sticker will probably say use BMW fluid OR Dexron III if BMW is not available.

If you need transmission fluid, after you figure out which one you have and the color of the plate - find a Transtar office. They are authorized ZF dealers. They will sell you 20-L of fluid for around $300. Sounds steep, but the dealership charges around $50/liter. They are also very helpful with transmission questions in general.