3GM30F with 1040 hours normally runs great with no smoke except at startup. I've just completed a short trip where I motored for 6 hours. During 3 of those hours I was towing a 30-ft sailboat with a very dirty bottom. During the trip I did not notice any smoking from the engine
. Once back in port I notice a dark stain flowing down from the exhaust
. I checked the oil
level and it appears to be the same as when I left. Do I need to be concerned about this and if so, what do I check?