The setup in the BMW is a little different to other makes that use the same transmission. In the BMW it doesn't totally lock, and fully unlocks much more often (when taking your foot of the gas for example). It shouldn't however being fluctuating when driving down the highway with constant throttle. It probably hasn't thrown a code as it isn't outside the tolerances yet. Only a matter of time.
Definitely a filter and oil change is the first thing to do. If this was my car, I would either do a filter and total oil flush, or do the filter, and then multiple oil changes over a short period. After 160k, the oil will be totally shot, and you want to get rid of as much of it as you can. The way clutches bind together in a wet clutch (ie. in an auto transmission) is very dependent on the oil. You will probably find the transmission drives much better as well. Regular fluid changes are without a doubt the best thing you can do to keep your transmission driving well for as long as possible.