Me too, iBob. I don’t understand how it works, but it does. I used slime in my bike tyres for yonks, mostly as a puncture preseal, but it also seemed to help with balance.
I have a mercury-filled Balancemaster fitted behind my prop and it seems to be running more smoothly.
My plane has no vibration monitor, but I have a habit of filling in my flight plan while warming up the engine. Going back through years of sheets lets me see trends in wobbly handwriting.