Ask a supplier who handles airconditioning and refrigeration compressors and parts. They will have compressor oil, which is pure mineral oil. Don't use too much though the motor windings are supposed to remain above the oil. That is why most folks believe you can't haul a refrigerator on its side.
BTW, next time you want to unclog a sump pump, start on the other end (the bottom) where the pumping takes place. I'll bet you know that by now; sorry we didn't get to talk before.