I change it 2-3 times a season........if u ever saw the inside of a lower unit, u wouldn't keep old oil in it......black means one or two things,,,it's old oil,,,,,,,,or when it was put together the clearence between the washers /rings etc. are to tite an it's frying the oil.....I think in your case it's just old oil...........smell it, if it smells burnt....have it checked out....my lower unit blew last summer from just ticking a sand bar,,1200.00 for a used one, parts alone to rebuild mine were 1200.00 never mind the 60.00 an hour for labor on top of the parts...everyone is so concerned about the power head but never give a second thought about the lower unit
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