What causes gear oil to turn black?

[Alumarine]

Drained the gear oil on my Merc 50 today and it came out totally black.
It was full I know that.

I thought I changed it last year but maybe not.
Even if I didn't it would only have about 20 hours on it.

What causes gear oil to turn black?

 

[jerryjerry05]

Re: What causes gear oil to turn black?

Using it will turn it black.The heat from the exhaust will help break it down.
Mercurys and Mercruisers I have found can make a stinking burned mess out of gear oil.
Keep it fresh.J

 

[Alumarine]

Re: What causes gear oil to turn black?

Thanks for that info. I'll make a point of checking it more often.

Have never had that problem with the mercruiser.

 

[Faztbullet]

Re: What causes gear oil to turn black?

I kinda disagree with the heat issue, yes it will break it down but usually if its black it been in a long time or wrong weight or cheap lube used(Autozone 80/90w is worst). I use Sierra synthenic (Volvo spec) in all my customers units I service in dry storage(214 of them) and rarely does it come out discolored much less black,sometimes its a clean as day installed.

 

[Alumarine]

Re: What causes gear oil to turn black?

I was wondering if it had something to do with the type of oil.

I bought the lower end used off a boat mechanic and don't know what kind of oil was in it.

I did put in the Merc High Performance lube this weekend so I'll see what happens next year.

 

[etech2008]

Re: What causes gear oil to turn black?

your on the right track cheap oil turns black fast I use merc high pro in commercial boats and guide boats that put more hours on there boat a year than we do in ten. The merc as well as thi sierra comes out brand new everytime.

black is usually from cheap oil.