85 HP Merc No gear oil

Year before last purchased 15 foot trihull with 77 Mercury 850 Thunderbolt 85 HP Motor. Motor runs fantastic, just touch the key and running.

That fall when it was time to winterize, a friend of mine who is into boats, (this is my first), told me to remove the large screw on the bottom left side (if looking at motor from the back), and check to see if there was any water there, or just oil. I removed it, and there was oil, and no sign of water, and maybe two drops of oil escaped and I put it back on tightly. He explained that if there was water in there, it would freeze over the winter and crack the foot of the motor. This past fall, I did all the same proceedures, (it ran great again all last summer), but when I removed the screw there was no sign of water or oil at all. What I can see inside looks clean and no rust or metal scrapings or anything, and it ran perfectly. Can anybody tell me where that oil could go, and what I should do next? I was about to put her in the water for the summer last weekend. Thanks!

Re: 85 HP Merc No gear oil

When you open the bottom screw last year, you should have gotten more than a couple of drops (Unless you did not open the top (vent) screw.

The first thing you want to check is the seal behind the prop. If fishing line gets in there, it will eat up the seal in a big hurry.

Did you ever notice any oil behind the boat when you were sitting still?

Once you replace the rear seal (I am betting this is were the oil went), check the manual for the correct weight and refill through the vent screw. Once it will not take any more, run it for several minutes then check it again.

ALSO, while you are down there, make sure there is no excessive play in the shaft.

Re: 85 HP Merc No gear oil

Thanks so much! I remember seeing some of that "rainbow effect" on the water when the motor was in neutral, I thought it was a bit of raw gas spitting out the exhaust or something, certainly could have been oil. Also, I didn't know anything about the vent screw, he probably didn't mention to open it on purpose so I didn't lose too much?!? He said I should see water right away if there was any because the oil would tend to float and the water would be at the bottom.
Please forgive my ignorance, chevy small blocks, no problem, boat motors, this is my first..not a clue. I will check all the things you suggested.

Thanks again.