Headed out to the Chesapeake Sunday to troll for rockfish (caught two keepers) and on the way back the ingine sounded like it caught a bag. I stopped and looked at the prop, no bag. It ran like it had no power. I noticed gray white grease or oil build up on the exhaust ports just below the head and at the exhaust port at the prop. I idled into the dock and went home. Last night I pulled the plugs and they were very dirty. I then checked compression in each cylinder, 130, 125, 130 and 130 PSI. Compression seems fine for a 1974 Mercury 50. I cleaned the plugs and put them back in the engine and it started fine, idled and ran at high rpms fine. I then decided to check the lower unit oil which looks to have been polluted. It is gray white like the oil coming out of the exhaust ports. I dropped the lower unit and will work on that tonight. I will drain the oil, take apart the water pump, remove the prop and during all this look for a bad seal. I was wondering if it is possible for lower unit oil to enter the compression cycle. When I pulled the plugs they were dirty but no signs of any water or oil foul, only normal black buildup from years of use so I do not think the water and oil is getting into the heads. Not to mention the compression test shows no signs of any gasket or head cracks. Anyone have any idea what is going on here? Things to look for? Thanks for your help.