i have a 1996 mercruiser 7.4 liter. The boat was recently purchased but has not been run since last summer. after taking it out for about 15 minutes, up to about 30 knots, back at the dock, i noticed a layer (probably 1/2 inch) of light brown sludge on top of the engine and some on the floor below it. Any idea what this is, how to take care of it? i'm pretty sure it wasn't there when i started the engine before the run. much appreciated.