That’s probably from using automotive motor oil back in the day. I was going to suggest cleaning it out and the bypass disc if it has one. The small holes in it get clogged with carbon making starting the motor a chore and poor idling. There’s no tell tale on these motors as you might lose the circulating water in the system if air can get in. It would be the same as a loose water tube connection. If you want to test the motor on a stand, you can undo the pipe plug under the cylinder and install an adapter for a garden hose. There’s no impellers in these motors to worry about!
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