First off, this is NOT to be confused with the following question:
What could happen if I used 4 cycle fuel in a 2 cycle engine?
2 cycle oil is used for my weed eater. There is no way I could use a whole gallon in a year.
Sometimes I run out of gasoline for my 4 cycle lawn mower, and that 2 cycle mix fuel is sitting there getting really old.
It will smoke badly and may indeed foul the plug(s). It will be less of a problem if you only use a SMALL amount of mix fuel in each tank, so the oil is more diluted.
IMHO, IME, if you use THAT little fuel in your weed whacker, you probably should sell it and buy an electric one, either corded or rechargeable.
If you don't want to do that, then you should invest in some good liquid measures that you will NOT use for anything else, and mix only a pint or quart of mixed fuel at one time - or purchase "canned pre-mix" fuel which typically comes in sealed metal quart cans - available in many places.
Edit WRT comments: I would personally choose an inexpensive graduated cylinder (or two) for the mixing - far less likely to be repurposed to food use by someone that does not know better than a tablespoon - also very easy to get a direct ratio in small quantity. But...2 Tbs in an ounce, 16 per cup, 32 per pint, 128 per half-gallon. So a pint of 32:1 mix is easy, but all my stuff is 50:1 mix. 256 tablespoons per gallon at 50:1 works back to one Tbs per 0.195 gallon (on a modern digital pump readout.) 40:1 is 0.156 and 32:1 is 0.125
You say mower in the body of your question, but the title could attract people looking for answer about adding it to their cars.
Your car or truck will easily consume leftover 2-stroke fuel. I typically don't do more than 1/2 gallon of 2-stroke fuel to 10 gallons in my tank. The small amount of bad gas and/or 2-stroke oil will be diluted. Your car doesn't expect perfect gas.
Sidenote: When I fill up my mower's gas can, I always add fuel stabilizer and an ethanol treatment. I then only fill up the weed eater and chainsaw's gas can from the mower's gas can so it also has those additives. I never know when my last use for a while will be for my chainsaw. With those additives, I don't have to worry about gas going bad in the tank or destroying the carb.
It will be 100% completely fine.
I have a 2 stroke racing off road dirtbike that requires 93 octane fuel and a 32:1 mix with racing oil. And frequently, that is the only gas can I have on hand because my other yard maintenance equipment runs on diesel.
I have a push mower that I use to do small areas of my yard, where the tractor with the 60 inch deck doesn't fit. Every time that push mower runs out of gas, I end up dumping the mixed 93 octane gas in it. And it runs fine. Starts fine too. Has never fouled a plug.
I do the same with my four stroke pressure washer. That always starts and has never fouled a plug.
The thing is that you're still putting gas in there. The oil mix ratio is pretty low, so while it is somewhat wasteful and unnecessary, it isn't hurting anything. Your valves will get a little extra lubrication but that's about it.
The two stroke oil will not hurt your four stroke mower in the least. Might make it last longer.
A 1:500 ratio will be fine to be used in a 4 stroke engine. As long as u add it to the fuel, not in the engine oil bay.
Look. As the Brit racer said, it hurts nothing. If you try to run a real heavy mix it will cause problems but a 2 cycle mix is just as he said, lubrication. Allot of drag racers go as far as to run mystery oil in their fuel as do I on my lawn equipment. 2 Stroke engines are actually way better lubricated than 4. Gas isn't that great of a lubricant but 4 Strokes are designed so you can run the same oil longer saving money in the end and be part of regular maintenance as opposed to burdening the owner with putting more oil in with every fill and having to figure out the right mix with every fuel stop with the amount of fuel you are putting in making it more convenient. It also helps so you don't have to shake your car every morning to get the mixture mixed back up instead of all the oil floating at the top.
That does not mean however you can just take a 4 stroke and mix your fuel oil together with out putting oil in the oil bucket. They aren't designed for that.
So I end this by saying put it in, you are doing nothing more than saving your piston rings allot of ware and tare and adding a little extra mileage while giving a little more punch for the fast lane on the highway and don't be afraid to add a small dose of Marvel Mystery oil in with it to make it smell a little better and just a little more slick with a little product called Sea Foam at least once a month to take care of carbon build up.
(P.S.) Marvel is also more ignitable than conventional motor oil as a stand alone as well so it will also help get a little burst out of it and help it burn a little more even.
I race a Lotus seven with a Hyabusa engine . All of us racers add a little 2 stroke oil to our tanks to extra lubricate the top end. It works great and protects the engine. It will not gum up your plugs or do any damage in small quantities.
We use half a litre to 45L of petrol. No smoke just added protection. A lot of diesel drivers say it improves mpg. It does make the engine run freer and smell nice (Castrol R). I have used it for 25 years. Just think about it. Its the same as expensive top end treatment by Wynns etc. Extra lubrication equals less friction equals more power and mpg. Simple science.
Just added 50:1 to my 4 cycle power washer (Honda Engine.) No visible smoke. Didn't see any difference in performance. I would think there is a tiny drop in performance, since 2% of the fuel isn't gasoline but high viscosity lubricating oil. I'm certain anyone can use 50:1 in their 4 stroke engines on an ocassional basis with no problem. In fact, the 4 stroke engine will pollute less burning 50:1 than a 2 stroke using the same fuel. This is because the valve system on a 4 stroke provides more complete burning.
You better never run 2 stroke on pure gas, though... That will quickly ruin the engine.
There is no difference between "2-cycle" and "4-cycle" fuel except that 2-cycle engines use the fuel as a lubricant, so oil has to be mixed in with the fuel (gasoline). 4-cycle engines are usually made to closer tolerances so having oil mixed with the fuel can gum them up and cause overheating.
As long as you only add oil to the 2-cycle machine's fuel, then you are fine.
Update, reading your question more closely I see you want to use the fuel mixed with oil. The answer is that it is not that great for a four cylinder engine, but it is doable.
