Another question I have: On starting the engine the other day, I noticed a little bit of a black slick on the water which seemed to be coming from the raw water discharge. I felt it and it seemed to be an oily carbon. I never noticed this before, although it might have been happening and I just didn't notice. After about 30 seconds or a minute it stopped. I figure it was an oil build-up or something like that. Is this right? Is it something to be concerned about?