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Mercury® Marine High-Performance Gear Lube from Basspro
If the horsepower of your Mercury outboard lower unit is 75 and below, SAE 80W90 Gear Lube is recommended.
Usually you fill it to the bottom of the hole.
Mercury Premium gear lube is caramel-clear in color. It should be changed every year or 100 hours of use. If it gets old and smelly it should be changed. If you see white streaks you have water leaking in through a bad seal. Mercury High performance Gear lube is blue/green-clear. See more at sterndrives.com
Quicksilver 80W-90 Premium Gear Lube
I had used a high performance gear lube quick silver marine lubricant it is not expensive and works very well
maybe
When you are looking for the object that is known as synthetic gear lube you can go to a store that is known as Wal-mart and buy gear lube for the car that you are driving.
The price for Gear lube varies, depending on the brand and they type of lube you are looking for. On amazon. com you can get a 1.25oz tube of Oster Gear Lube Clipper Grease for under $5.00. You can also get a tube Sier Syntletic Gear Lube Volv0 can run you $185.00.
The Quicksilver high performance lube will work perfectly fine in a 40 hp Honda. This lube was formulated for use in larger, high demand gearcases and outdrives. The only precaution to note is not to mix this lube with other types of gear lubricant.
answered on dootalk. 75W90 synthetic gear lube
For the 80 hp Mercury, you may use any brand of gear oil that has been blended for use as an outboard motor gear lube. There are many brands available other than oem, and are specially formulated for outboard use.