Port Engine losing oil?

[dougl33]

My port Merc 454 started using oil for the first time in the 3 years I've owned the boat. So far its used about 1/2 a quart in about 35 hours. I know this may not seem like much, but I haven't had to add a drop of oil to either engine the first 2 years I owned the boat. Total hours on both engines are around 1125. I put pads in the bilge under the engine to see if I could figure out where it was leaking. I have seen a couple of small spots, but nothing that woud add up to a 1/2 quart. Also, it does not look or smell like its burning oil. I was reading a post on another site by an individual who had a similar problem. He was seeing an oil slick in the water behind the boat when running his engine. Someone else posted and told him that he had had a similar problem a few years back and it turned out to be that the oil cooler had developed an interal leak and was sending oil out through the exhaust. I did notice that the topsides just below the upper exhaust outlet are a little stained. Has anyone else ever had this problem? Thanks in advance for your advice. Regards, Doug L. 1986 33 Bertram FBC Queen Elizabeth