I purchased a boat with an old mercury mark 6. It was missing the pull cable. I replaced it and got it started. I noticed it wasn't pumping water out the 4 holes half way up the shaft, so I started to investigate. Is the open hole in this picture the drain plug for the oil in the lower unit? What type of oil should I put in the lower unit? Any tips on taking it apart to see what needs to be replaced to get the impeller doing its job?