Not paying attention, I put 32oz of lower unit oil (80W-90) in my engine. (20hp Merc EFI outboard)

The motor capacity is 1.1 quart of engine oil....so I drained the gear lube out overnight. I also changed the filter beforehand but obviously didn't run the motor so it should be OK.

Any other precautions or is that enough?

MAGA!

Put something like Sea foam in it to flush it out. More than one can. Maybe Berryman's B-12.

That's all I got.

Measure what you drained out ?

drain it and fill it up with the right oil, a little gear oil won't hurt it.

This ^^^^^^^^^^^^^

Don't do something dumb, like forget to tighten clamps and put on safety strap before starting.

Maybe do the drain and fill thing then run the engine an hour and drain and fill correctly if you're worried

If still worried the small amount of gear oil left in it. Run the engine to operating temp, drain the oil hot.

^ Thats all, if you want a little extra peace of mind. Like STX mentioned dont worry about it after. a little isnt a worry.

I don't know crap about boats or their needs, but if that gear oil stinks (has sulphur)
I'd do as Pat says. If the drained hot oil smell like gear oil, I'd do it again.

Don't know if it would matter, just know oil is a hell of a lot cheaper than engines.
One thing I do know about boats, in a world of crazy prices anymore, boat stuff is
cutting off your own Johnson crazy.

Your oil pump might bog a bit if it gets an undiluted slug of gear oil... You ight refill it with something from the light end of the recommended range.


Any oil is better than no oil.

Yep

It's 1 qt of oil. If I was the least bit worried about it I'd put the right oil in without running it, then drain it out to flush any remaining gear oil. Fill it back up and run it.

Way back in the day, had a big block Plymouth that "used a little oil". Solution to slow the oil consumption was to put in 4 qts of straight 50W racing oil. Then top off with 90W gear grease. Cranked slow in cold weather. Otherwise zero ill effects.

Fill it with the proper oil and run it on Idle for 10 minutes or so, drain it, put the proper oil in it and spend a little time reading the bottles in your garage.........

It probably won't hurt it, but I'd fill it with diesel and let soak for a good bit, then drain it. Let it drip for an hour or more. Then fill it with the right stuff.

Add a few ounces of Diesel fuel or Mineral Spirits to a regular oil fill, run to operating temp, drain and refill. On the sludged-up antique engines I work with, I overfill the crankcase with 3 quarts of Diesel on top of a full fill of cheap oil, and run the engine for 20 minutes at fast idle. It cleans up the sludge like a washing machine.

DO NOT PUT ANY LOAD ON THE ENGINE WITH THE SOLVENT ONBOARD! fast idle only at no load.

Small engine have small delicate moving parts that may not like the added stress of thick oil. I'd just want to be sure I got a high percentage of it out of there before running.

A new 20 HP Merc is about $3600 BTW.

No kidding, if a 20HP tiller handle long shaft Mercury can be had for that I will buy one. My net boat has been without a motor for over a year. It is 18 feet long and about 6 feet wide at the gunwales. Don't need to go fast, 10 or 15 MPH is fine

1 qt, yes, but is it 100% of the oil in the engine? If it holds 4 qt, that's something else entirely.
I wouldn't worry about it either way. Just drain it and refill. Run it a bit to absorb whatever's left, then change it again.

Any brass in the engine?

I used to add a quart of Rislone every 2 or 3 oil changes

This is what I was thinking.

If you don't want to use diesel put a qt of transmission fluid in there, let it sit a bit then drain.

Fill with proper oil, let idle until up to temp then drain and start fresh.

Unless it is a modern engine that take 0W-16... You should be fine with a drain and refill.

Edit: My bad didn't read boat.

www.onlineoutboards.com

Why not call a Mercury dealer ?

Check these guys. Best prices I could find. Be careful, two or three years ago there was an almost identical scam website which demanded payment via cash app.

https://www.cumberlandwatersports.com/

Small Mercury engines are built in Japan by Tohatsu. Engines by either name are identical except for decals and price. I purchased the EFI, electric start, power tilt, 20 hp Tohatsu from them two years ago and was pleased with everything about the transaction.

Tohatsu 2024 Tohatsu 20 HP MFS20EEFL
20hp Horsepower
106.0 lb Weight
20" (Long) Shaft Length
$3,330.00

Mercury 2024 Mercury 20 HP EFI 20MLH
20hp Horsepower
107.0 lb Weight
20" (Long) Shaft Length
$3,520.00

Mercury owns Tohatsu. Also, Nissan motors are just rebranded Tohatsu's. All parts are said to be interchangeable. I don't know if Merc, Tohatsu, and Nissan are all the same. I read that Honda also has some kind of arrangement with Tohatsu but I don't know if Hondas are made by them.

It is the other way round with Honda/Tohatsu.

Honda started building 60hp - 250hp for Tohatsu circa 2014.

https://www.boatingmag.com/boats/honda-to-private-label-large-outboards-tohatsu/

We bought a Merc, I don't know of a Tohmatsu dealer within a hundred miles of where we fish

Old Hardleys leaked so bad we always ran straight 50 weight in them .

At which point did you realize you added gear oil instead of engine oil? Was it before or after it took 15 minutes for the bottle to drain?

Drain it. Plug it pour one quart of the correct oil in wait a few, drain it. Fill it, run it, forget about it. That gear oil won’t hurt a thing.

Jim Conrad’s question is in reference as to the additive that’s in oil packages. Some have adverse effects on yellow iron if you will. Hope he corrects me if I’m wrong in the assumption.
80W/90 gear oil has the same viscosity as straight 50 weight motor oil.
All that said I would drain, refill, run. Change oil again at half interval and then back on schedule.

I didn’t read the last couple of pages. I’d drain it. Throw some cheap 10-30wt and about 3/4 of a qt of diesel, let it idle to warm up good. Drain it, put your good oil and new filter in it and drive it.

Nope, just fill according to the factory specs. Most the time that's fill it from the bottom, until lube comes out of the top hole, then screw in the top one and quickly put the drain screw in. I never measure mine, but some newer motors might have different requirements from to old type motors.

That's for lower unit.

He is talking about engine oil.

This but when you add the seafoam I'd top off with the correct oil. Run it a few minutes, then drain it again - overnight if you are anxious.

Then refill with the correct oil and yer off to the races....

I run a number of motors ranging from 6hp 2 stroke to 250hp 4 stroke, so empathize....

About a half a century ago a mechanic told me that 90w gear oil attacks brass. I don't know what the internal workings of a boat motor are like. Hence my Sea foam thought. Some solvent to get the gear oil out. Hope it works out for you OP.

I quart of oil is cheap. Drain the heavy stuff, fill with one quart of oil, take out spark plug, spin it over several times, waiting a few minutes between each spin time to lessen stress on starter, drain, fill with new oil, put in spark plug, start, run until hot and drain again, change filter and add oil, run normal.

I’d add regular oil back in.

Turn it over a number of times. Then I’d drain it. Might do that again depending on if you smell any hint of gear oil in the drained oil.

After that, I’d fill it and run it briefly in neutral. Then drain and refill.

All these variations on a theme…probably be okay with any of them. What I would do: drain the l/u oil, dump in a quart of Marvel Mystery oil, unhook the spark plug(s), turn it over for a bit, drain it, refill with correct oil, run for a while, drain and refill.

Maybe kind of a belts and suspenders approach, but I have a 22 year old 90 hsp Yamaha 2 stroke that still runs like a champ and has never given me a problem

Drain it. Put in cheap oil. Run it a bit. Drain that. Cheap oil again. Run it a bit. As in idle it until temps come up or maybe 15 minutes or so. Drain and put a new filter on and the right oil.

We used to drain old oil in grandpas dodge truck. Dump in a gallon of diesel. Run it for 30 seconds or so IIRC, drain and put new oil in. Truck ran a LOT of miles for an old engine. The flush must have never hurt.

But I dont' think you have to here. I suspect just a straight drain and refill with oil and run and the dilution numbers will be so insignificant that would be way more than enough anyway. But the change oil twice is what we do with a motor that went under water.

Did everyone finally figure this out? lol

There’s no way this really happened.

Drain it and fill it with the proper oil and go on. When my son swamped my 4 wheeler, we drained the oil, pulled the plugs and poured diesel through it to get the moisture out. Since you have oil in it and not water, just draining and refilling with proper oil should be fine.

Final procedure is awaiting NASA Spacewalk Planning Group's approval.

Sounds like a government operation................................................................

It should be fine. Drain it. Fill it with the correct oil.