Plastic type reservoirs are often ATF, the CHF11S one has a sticker (a green ring) on the top. CHF7.1 ones have a yellow ring (Beware: they come off with age). e34s with metal reservoirs (almost... ) always use CHF11S. e32 had more exceptions to this rule than e34. e23/24/26/28 I don't think used CHF11S much (but there were some with LAD whihc would have. Anything with LAD or SLS will use CHF11S and share a modified power steering pump, as it is best to use the one very high pressure hydraulic circuit (which is why they use CHF11S). From BMW's POV, the only expensive bit in this approach is the multi-purpose HP Pentosin oil.
Since you are not using the reccommended fluid, you should try swapping your synthetic ATF (is that what you are using- not synthetic something else?) to Mercon or Dexron.
I don't think it can be your steering box. Pump maybe... but I'd be surprised if it was. You can probably get a $20 kit for it from www.pskits.com.
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