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I am assuming you mean the vent in the lower unit and mean gear oil?
Take the bottom drain screw loose on the lower unit. Drain about a teaspoon of gear oil out then tighten the plug. If you get water immediately drain the gearcase.
Compare the gear oil you drained with what came out of the vent. Is it the same?
If it is different it may be unburned fuel washing down the midsection.
Check your spark plugs to see if one cylinder is not firing, cleaner than all the rest and do a compression test.
Post back what you find and we can go from there.