Lower Unit Gear Lube Swap

If you’re a River Rat, you probably have an outboard motor.  Are you keeping it maintained? My outboard is something like 28 years old and still runs well.  I can’t claim it’s history though. Only that they can run a long time. This is a quick post on changing your lower unit gear oil.  

I try to use the best of the best synthetic lubricants I can.  This Amsoil marine gear lube is claimed to keep it’s lubricating properties even with 10% water in it.  It isn’t cheap though.

My 15hp Evinrude specifies 9oz of lube.  9.9hp and 15hp Evinrude and Johnsons through the 70s, 80s, and 90s all had the same lower unit.  I’m not sure on the other manufacturers, but they are close. This tube is a 10oz size designed for these small outboards.

The process is simple.  Pull the fill plug and then drain plug.  The fill lets the lube flow out easily. Filling it is a pain in the ass though.  You can get a quart size bottle of lube with a small hand pump using a small thread in adapter and pump the lube up through the drain hole.  Install the fill plug, then quickly unscrew the adapter and put in the plug. Supposedly the fill plug creates a small vacuum so you lose very little.


Or you can do like I did and just put the spout of the 10oz tube in the fill hole and squeeze it in.  It doesn’t have a vent so can be tricky sometimes to get the lube in. Some people claim you can’t fill it right.  But I seemed to get all in but 1oz or 9 out of 10. Which is exactly what the spec is.


Do not mess with the phillips screw.  It is used as an adjustment point in the lower unit gears.  Remember that both drain and fill have the same heads, large straight slot.  The large phillips is a separate function.I seemded to notice a .5-1mph increase after this lube swap.  Keep those outboards singing. Given that I’m a little OCD, I’ll probably change it again this fall and then let it go for a year.


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