[Bipolar]

I used to own a 1985 BMW K100 RT It had a handlebar vibration at highway speed that I found really annoying. The fix was to remove one handlebar end weight and fill the handlebars with oil. It worked quite well to reduce the vibration.

 

[Bipolar]

I don't recall what oil I put in the handlebars. It would have been what was available in my shop at the time so most likely engine oil.