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Lower unit oil change

maxthrottle

Contributing Member
I pulled the boat today and changed the engine and lower unit oil. The starboard engine lower unit oil was black, no foam or evidence of water, the port engine oil was gold, just like a new bottle, but maybe just a pint, not a quart. I put about 100 hours on the engines these season, just shocked to see the differnece in the color and amount of lower unit oil. Any ideas as why I am seeing this amazing difference?
Thanks,
Bob
 
Some gear cases do that for unexplained reasons. I had a BF225 what would always turn the oil black within 50hrs, I now have a BF 200 on the same boat and the oil looks brand new after 100hrs. As long as there is no metal on the drain plug magnet I wouldn't be concerned. Just one comment though, Mercury high performance gear lube is the best gear oil around and I would be using that instead of the cheap synthetic blends.
 
I view black gear oil as a sign of excessive friction and or heat but there's no combustion taking place in the gearcase. The reason I say and/or is that, obviously, friction produces plenty of heat but another source of heat in an outboard gear set is cavitation. Cavitation produces enormous heat and some has to be transferred to the gear-case.
Are your engines counter rotating or both right hand rotation? If they both spin the same direction then one will always be working differently from the other as the vessel is counter steered to overcome the torque at the stern. This may set up some cavitation conditions for one of the props.

Either way, if no metal particles were present in the drain oil then drain and refill is indicated with possibly a shorter interval to check up on things. It may be a condition that you have.to, and can, live with.

As far as the low oil level, it was either under filled at last service or some has escaped. Only a case pressure test would answer that question.

Good luck.
 
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