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gunflint

Lower Unit Lube

4 posts in this topic

What's The difference between Marine Lower Unit oil and a heavy duty synthetic 75-90?

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Regular 75-90 does not have the required additives to allow for the high speeds of the inlet shaft. 75-90 is what I'll call a "Low speed" gear oil. Without the additives, regular 75-90 will foam due to the high speeds involved in the input shaft.

Foam in any lubrication system is bad, very bad.

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The way one mechanic explained it to me was:

Marine lube is a nonfoaming formula, your LU is a sealed system that is almost completely filled with lube. There is no place for the lube to to go if it foams and expands so it would blow out a seal.

Sounded good enough to me that I've only used marine gear lube for about 30 years.

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