This is an odd question but here goes. Last winter I had the oil changed in my Cummins 5.9TA's but was not present when this was done. When I got back to the boat 2 months later I checked the oil and the oil in the port engine looked clean while the oil in the starboard engine was dark. I mentioned this to the mechanic and he said it was due to the old oil still being in the oil tube and this was what was showing on the dipstick. We have only ran the boat a few times since for only a couple hours of run time but when checking the oil a couple of weeks ago the oil in the port engine still looks relatively clean and the oil in the SB engine still looks dark. I have the oil change system but not sure if this has anything to do with it. Did the mechanic only do one engine? He did get side tracked in the middle of winterizing it and maybe forgot what he was doing. Has anyone encountered the dark oil in one engine after an oil change? Thanks!!