Lubricating the flex shaft

I’ve watched the bur lube debate and have seen several suggestions
to use a bottle cap with cotton (sponge for those that can’t wait for
the flex shaft to stop) and oil. What kind of oil do you use?

I keep oil of wintergreen handy, in a small metal watch parts tin
with a sponge for dipping burs, gravers, drills etc. (No am not
patient enough to let the bur stop spinning!)

What kind of oil do you use? 

Vaseline or non graphite grease

Stephen Wyrick, CMBJ
Gemmologist