Re: Water in the Lower Unit Gear Oil on Mercury 75hp Outboard (2 stroke 4cyl) Serial# A953769
I tend to take a slightly different approach.
Milky gear oil does indicate water intrusion. But if you live in a 4 season location (Canada, Northern U.S.) you are almost always going to have some water intrusion. 1/4 of the year the gearcase gets baked in the hot sun (which causes everything to expand - at different rates) and then all winter its subjected to negative temps, contracting everything up. So you will never have a "perfect" seal.
I would drain your gearcase and install new. Keep some of what you drained in a glass jar for a day or so and see if water actually separates out. If it stays milky but you don't get a layer of water on the bottom, your seals are probably ok. If you see a layer of water then you probably have a real "leak" as opposed to just some seepage due to the expansion/contraction of the case.
If it's just seepage, then simply keeping the oil changed once a season (and best done just before you winterize) is usually sufficient. If you boat in excess of 100 hours a year, then an oil change mid way through the season is also in order.
If it is a leak, as Racerone suggests, get a complete seal kit...
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