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When you say seal, are you talking about your rubber gasket for your fill and drain screws? Or your oil seal behind your prop?
If it were me, I’d probably do just what you said. Check it after a couple of trips and if you don’t see milky oil then it likely was just a failed gasket on one of your screws (assuming that’s what you meant). If it’s that black every time you change it out, I’d probably change the frequency that you are replacing the gear oil. I think most manufacturers say 100 hours.
