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Maybe this is a stupid question (let me know if it is). I am prepping my (new to me last summer) boat for launch for the first time and getting the outboard (a 1984 7.5hp Mercury) ready for the season is part of the process.

When I drained the lower unit back in the fall after haulout, out came (eventually) this fairly thick grey sludge. My initial reaction, was: "wow, it really must have been awhile since this stuff was last replaced; either that or a lot of water is getting in here", because the Mercury Marine lower unit oil that I had just bought was much lighter in consistency and was more of a blue colour.

Anyway, I've since noticed that there are heavier weights available for outboard lower unit oil/grease. Have I put the wrong stuff in my motor?
 

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Well

Merc only sells dino or synthetic oil and you need to have the lower unit pressure checked becasue it has a leak


Also do not reuse the gaskets on the drain and fill level plug they are a ONE time item and will let in water
 

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This is what they recommend .

"You can use any brand of lubricant that has a grade designation of API GL5 SAE 80 or 90 weight. Make sure you do not mix different oil brands or weights."
BTW - if you had water intrusion into the lower unit, the oil would be a milky colour. Your dark oil suggests that the former owner neglected to change the lower unit oil on a yearly basis.
 
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