Oils have definitely improved in the last hundred years, with the
possible exception of sperm oil, which is, of course, not available
today. Synthetic oils have better thin-film lubricating properties, do
not oxidize as quickly (maybe even never) and do not absorb moisture as
do other oils.
It is not a good idea to mix oils, although most synthetic oils claim
to be compatible with conventional oils. Still, I would remove all
traces of the old oil and use a good synthetic oil of suitable
viscosity. If a synthetic oil made specifically for electric motors
cannot be found, an automotive engine oil should be fine (others may
know of reasons that this is not a good idea).
I have been using various brands of oil obtained from bicycle shops. I
buy the general purpose oils, not the ones for chains, as these are
really gummy and might not flow into tight spaces.
Whatever you do, do not use 3-in-1 oil or let WD-40 get anywhere near
your motor.
Spencer Chase
Garberville, CA
http://www.spencerserolls.com
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