Hi Touchrain,
There are a few things I do on our E30+ to keep the wheel turning smoothly, and they are likely similar on your E38. Most important I think is to liberally grease the rudder post--the top zerc fitting is hard to see on ours, almost behind the post and the lower one is underneath the floorboard which is held in by three screws, but it is there. Next, oil lubrication of the cable under the cockpit floor helps. I also check cable tension--if too tight the wheel won't turn easily; if too loose, the cables could slip off the sheaves. While down there I check to ensure the two sheaves are running smoothly and I lubricate the bushings there. I understand that these bushings can wear, but I don't have any experience with that, but that could be part of your problem. Next I remove the compass on the pedestal and lubricate the chain in the pedestal with oil, being careful not to get any oil on the area where the brake pads rest. Finally, I spray some silicone lubricant in the gap/joint where the wheel shaft enters the pedestal, but not too much to not get it on the brake pads inside.
If you've done all that and the wheel/rudder still doesn't turn as it should, is it possible that you have a bent rudder post or similar problem?
Good luck, hope you get it solved!
Frank