Years ago, when I still had a job, I was taught to never use automotive
motor oil in electric motors.  It is formulated for a totally different
purpose with anti-foaming agents, detergents, and many other
ingredients.  In an electric motor, no oil is consumed, and losses are
from evaporation, and the additives stay behind, leaving a sticky mess.

Use non-detergent 10 or 20 weight oil only.

Hal Klassen