mercury, 90elpto, 2005

by Peter Nichols
(Essex, MD)

I found milky oil in my lower unit along with a good bit of black oil. There was no actual clear water in the flow when draining. It looks like the bottom drain plug did not have a washer with the plug, but some blue sealant instead.
I'm going to pressure test to see if the rear prop shaft seals are leaking (they look good). I will also check the water pump seals on the shaft for leakage. I'd rather not take the whole thing apart.
My question is:
1. If the plug was letting water into the unit, can I simply replace the oil or should I flush out any residual milkshake? What can I use to flush? Would diesel or lacquer thinner do the job or hurt anything?
2. Does the lack of clear water in the drain out indicate that only a small amount of water came into the unit via the drain plug and caused the color change?
3. What does the black oil indicate? The motor sat in the down position all winter.

Thanks for your help!