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