An inappropriate choice of lubricant
May 8, 2012 8:48 AM   Subscribe

I put two-stroke oil into my four-stroke lawn mower engine. How bad is that?

I mistakenly used two-stroke (2T) oil (unmixed with gas) to fill the oil reservoir on the lawn mower. It ran fine, and started easier than usual, I'd say. How immediately should I replace the oil with the conventional 30-wt oil actually specified by the engine? Is there a big viscosity difference that will damage the engine if I leave the two-stroke oil in it?
posted by cardboard to Technology (9 answers total)
There's a huge viscosity difference. Do it ASAP. If you like how easy it started you could use 5W-30.
posted by thebigdeadwaltz at 9:18 AM on May 8, 2012

Does the container of two-stroke oil have any indicative markings? It probably won't have a weight (viscosity) marking, but it might have other industry group markings on it.
posted by the man of twists and turns at 9:19 AM on May 8, 2012

This question sent me googling all around because I was curious. This is definitely a better situation than putting 2-stroke GAS in a 4-stroke engine.

The general answer is 2-stroke oil is regular oil that's ~20-weight, with lots of additives to make it bio-degrade and burn cleanly (ie not foul up engine/plugs). There's additional commentary about the temperature sensitivity of the 2-stroke oil for the engine it's used in, so it may also breakdown differently in a 4-stroke engine.
posted by k5.user at 9:36 AM on May 8, 2012

Even if it's the correct, or correct-ish viscosity, it's made to burn, and will likely burn off. I'd swap it out immediately. A lawnmower crankcase is what, a pint?
posted by notsnot at 9:48 AM on May 8, 2012 [2 favorites]

I'd swap it out, mostly because you don't know for sure what the viscosity is and it doesn't have the friction modifiers and detergents that engine oil does. Small engines are surprisingly durable, so you're probably all right as far as bearing wear, but I wouldn't run it again until you've at least changed it.

k5.user: If by 2-stroke gas you mean gas with 2-stroke oil mixed in (premix), you're incorrect. I ran my mower off VP C-12 premix with Maxima 927 at 32:1 (for that rich beany smell) for years because it was leftovers from my race bike. As long as you don't have so much oil in there that it leans the motor out excessively, lawnmowers and roto-tillers run fine with on premix. Anything with an emissions package on it (catalytic converter, O2 sensors, etc) will have serious problems, but it's not a matter of two stroke vs. four stroke motors.
posted by hackwolf at 12:33 PM on May 8, 2012

I speak from indirect experience on a troy-built chipper/shredded that was a 4-stroke engine that my family put gas/oil mix (2-stroke) in. Engine smoked a whole lot, then died. Dealer put a new engine on.

Might have been a matter of swapping plug, poking the carb and running a shedload of carb/choke cleaner through it, but I wasn't there at the time.

So it may be a YMMV depending on engine type and application and should fall under the "generally not advised" heading.
posted by k5.user at 1:18 PM on May 8, 2012

There's no real reason to take chances - do another oil change, using the right oil, ASAP. I would do it before I cranked it again.
posted by randomkeystrike at 1:23 PM on May 8, 2012

I would absolutely, for sure correct this before starting it again. And I would also flush it out with some regular oil before refilling it. I would assume you've done some level of damage to the engine already, but I'm nowhere near knowledgeable to quantify that.
posted by LowellLarson at 2:23 PM on May 8, 2012

As long as the mower was running OK and there were no outward indications of problems (extra noise, running hot, etc) I see no reason to assume that there was any damage to the engine. From what I know, premix uses the same SAE30 base as engine oil, just with a different additives package. Compare, for example, the technical specs on this type of four-stroke oil and the this premix oil, both from the same company.

k5.user, I think it's highly likely that there was something else going on there, or you were running a really low oil:gas ratio or something. Using old premix in lawn equipment is something that I've done for fifteen years, and is done so frequently by racers that running your mower on high-test is a "you know you're a racer when" joke that's older than I am.

There are only a few ways premix can harm your four-stroke engine, as far as I know:
1) If the mix is too thick, your engine will run lean because it's not getting enough fuel; oil is lubrication, not fuel, even though it does combust. This will cause your engine to overheat, damaging all kinds of things. Most lawn equipment has pretty broad tolerances for the fuel ratio, so unless you're running an old-school race mix, something like 12:1 (most chainsaws and weedwackers run 50:1), you should not be at risk for this.

2) If you let it sit in the tank or carb for a long period, the oil can adhere to parts and gum up the carb, requiring a rebuild.

3) Castor-based premix, when subject to low temperatures, can separate significantly, leaving you trying to run on nothing but oil. This will not get you very far. Synthetic premix generally does not separate and is more stable.

Again, premix is generally safe to run in your small four-stroke engine as an occasional thing. Don't put it in your car or anything with an emissions system, use it right away, and don't use it when it's freezing outside, but it is definitely not worse for your engine than using 2-stroke oil as an engine oil.
posted by hackwolf at 7:11 PM on May 8, 2012

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