Replaced my regular and boring E60 steering wheel with a modified m sport steering wheel a few years ago. After the install the steering wheel has been squealing/ squeaking whenever I made turns. It got so bad and often I finally got fed up and removed the wheel to try and find out what went wrong.

At first I thought maybe part of the leather coverings behind the steering wheel wasn't fit flush and is rubbing against the steering column, but close inspection verified that the leather wrapping on the wheel is fitted very well onto the wheel frame.
Upon closer examination I found that the parts that touches between the wheel and the steering column are the soft touch material that felt slightly sticky to the touch.
So I opened up a can of clear 3M silicon grease and applied both on the wheel and on the steering column, on the parts that I know will be in close proximity once the wheel is installed.
Sure enough, after the lube is done, there's no more squealing from the steering wheel!

 

Great tip! I've moved your post to the DIY section so others looking for a solution can find it easier.