I had a 15 hp OMC I got recently and there was lots of oil coming out of the top lower unit shaft seal under the waterpump...I mean a lot of oil. In the water or not it leaked a lot. If it is a seal and in those old 18's it can be it may be the prop shaft seal or the upper drive shaft seal, or just exhaust oil as mentioned above. It also can be the shift shaft seal, but these seem to be a problem less often and that is good as they are the most hassle to fool with.