black lower unit oil

Does anyone use Amsoil lower unit oil in their outboard? When I changed mine this year it was as black as diesel engine oil when it gets changed.It smelled normal and didn’t see any metal in it.It has been in for two years because I only put about 20 hours on the year before and total about 50 when changed.Never seen lower unit oil this color when changing in the past.Drained correct amount out when changing.Not sure if this is a problem or not.

What outboard are you running, if running a 3.0 Optimax they require a specific lower unit oil or they run very hot.

Is this the 1st time you’ve run synthetic lube?
I know the 1st time I drained synthetic from my LU it was very dark. Since then, it looks as good as new when I drain it. I thought there must have been a better cleaning additive in the Amsoil.

I don’t know the formulation, but I do believe Amsoil as well as other synthetic lubes do use more detergents. So as mentioned by Pat K, the lube is cleaning more of the deposits out of your lower unit that were left behind by the previous lube. If you want to, you could do a quick flush with diesel or kerosene, that should remove most residue, then refill as usual. The next change should be much cleaner.
Always use marine-rated gear lube in your lower unit, and watch for any sign of water when you drain it – milky lube or water/oil separation in your drain pan. Between changes, watch for any sign of oil residue on the outside of your lower unit that could indicate oil leaking out, a sure sign of a bad seal. Remove fishing line that gets around your prop/shaft immediately, one of the worst culprits for damaging seals.

I’ve seen the same when using Amsoil before. They said it’s from the detergents they use that clean out the old crap.

All is good if you didn’t have any water or metal chunks coming out

Thanks for the responses. It is a 2017 Merc 150 4 stroke and yes it was first time with Amsoil. I guess that makes sense if there are more detergents in it.Yes, always use marine gear lube. I refilled with Amsoil and will run it a while ane drain a little out to see whats there. Might even just change it again in 10-20 hours just for piece of mind. Thanks.

I use Amsoil lower unit oil and mine come out black every year. From what I’ve read that’s ok as long as there aren’t metal shards or milky looking watery oil coming out.

Without doing some flushing of the gear case, there will be dirty residue left behind each time you drain the lube, sticks to all surfaces – bearings, bushings, housing, gears, etc. Not going to cause damage, but cleaner lube makes it easier to notice any stuff that doesn’t belong in there.
Add enough diesel or kerosene to mostly fill the gear case through the fill hole with the drain plug in, shake it around, rotate prop shaft, shake some more, then let it soak/settle for a few minutes, then drain. Add some lube, rotate prop shaft a few revolutions, drain that lube, then refill as normal. Not necessary, but can give some peace of mind.

mahtofire14 wrote:

I use Amsoil lower unit oil and mine come out black every year. From what I’ve read that’s ok as long as there aren’t metal shards or milky looking watery oil coming out.

2x. Mine comes out dark every year out of the F250. I looks like new when drained from my 9.9.