Lower Unit Oil Dark, almost Black

[OOPSMEN]

Why all of a sudden when I changed the oil in the lower unit this past fall it was almost black, with a smell, almost like exhaust?? 1995 Johnson Fast Strike 150. I do mix the oil in with the gas, a few times I did get carried away with the oil, could have this caused it?? Found no metal shavings, or water in the unit, no leakage. Using Pennzoil lower unit oil. Any suggestions.