5W20 oil viscosity issue

My Snapper mower has a 11.5 hp B&S OHV engine that was filled with 5W20 oil for storage over the winter to enable
me to start it occasionally in the cold weather to keep the fuel system from gumming up. Today I fired it up and decided
to cut the yard but after 10 minutes the engine began to smoke and lose power, eventually stalling. The oil level was OK
and after leaving it to cool a few minutes I checked the plug and it was loaded with oil so I changed it and the engine
started but smoked. I drove it to my shop and adjusted the valves and drained the oil which ran out like water. After filling with
HD 30 oil it started on the first pull and I finished mowing the yard, about half an hour, with no problem.

The light oil was apparently too thin for the air cooled engine and B&S does specify 30 weight when the temperature is above 40*.
Hopefully there was no damage done to the engine. I will be much more careful about the oil viscosity from now on.

Why would u use 20w when motor is spec’d for 30w? I used 5-30 syn oil in my snowblower and it runs fine. I had issues with poor starting which turned out to e carb related and used syn to make starting slightly easier. Now that carb is fixed I used reg 30w last winter and it still runs ok. Only used it for 5hrs or less the entire winter. So not sure if longevity is compromised by using 5-30.

I would never run the lawn tractor with 5W20. Try the 15W40 for winter storage next time. Believe me, it’s thinner than straight 30.

I will give my usual advice. Follow the instructions given by the manufacturer of the product, no matter if it is a Sherman tank or a Weed Eater.

I can see why you might use 10W-30 or 5W-30 oil in a pinch, but using a 20 weight oil in an engine that requires 30 weight oil is asking for trouble. The results you got don’t surprise me at all.

The light oil was installed for winter storage…

You were very close to seizing the motor. It severely overheated and the internal friction got very high producing more heat. I’d expect you did damage the motor, but if it runs not much you can do about it now.

Really if you use Stabil, I use the blue stuff for ethanol treatment, there is no need for any winter start ups so you can change and refill with straight 30W year round.

Yes, @Uncle Turbo. It is likely that some damage was done. Hopefully it is not significant. And although I use Sta-Bil in the gasoline that is kept in containers for the lawn mowers I try to make a habit of starting the engines every 2 weeks and that 11.5 B&S was a particularly tough engine to crank when cold so I installed the light oil and starting was easier. The idea was to cut the grass and then drain the oil while it was hot. With all the improvements in oil quality I thought that the multi viscosity W20 was adequate. Obviously it wasn’t. The oil ran out like draining a bath tub. It was so thin that I checked for gasoline contamination.

5W-20, or 5W-40 is basically 5W oil as the base with additives to keep the oil thick when at operating temperature. The issue is operating temperature is figured for a water cooled engine, not an air cooled one. Air cooled engines can run much hotter. That can overwhelm and break down the thickener additives and you are left with 5W oil.

I think the starting every couple of weeks is not necessary, but that is what you want to do. Any good straight 30W oil should be fine at this point, but you might consider a 30W oil spec’d for small motors which is has an additive package designed specifically for air cooled engines.

There seems to be a misconception by some people. It’s not 20 weight, it’s 5 weight he put in there for the winter. I do a similar thing but make sure to change it out in the spring. So far I’ve been lucky to remember to do it.

The 5W20 indicates it is a 5 weight oil that will thin no more than a 20 weight would at operating temperature.

So there are two issues using this type of oil in a small displacement, air cooled engine; 1. the oil is significantly thinner than it should be while it is warming up, allowing oil to slip past the rings. B. the oil has viscosity improvers added to it to give it the multi-vis properties. Not much of an issue in a water-cooled engine with a large oil reservior. In this little engine, it gets far hotter, has less oil capacity to begin with and needs all of the lubricating ability it can get from the limited amount of oil it has to work with in the first place. VIs are not lubricants, they displace actual oil in the volume of oil you put in the engine. The higher the viscosity spread, the more VIs are in the oil and displacing the actual lubricant. And those VIs break down with heat and can cause all sorts of problems in air cooled engines.

I have begun running synthetic diesel oil in all my non-emissions engines. I have run 5W40 Rotella T6 without any issues. I am considering going to the 10W40 or 15W40 version of this oil for summer usage. This is good oil intended for hard working diesel engines so I figure it is also good to use in air-cooled engines. I talked to a couple shops that do this same thing when changing customers’ oil in lawn equipment. Air cooled engines run much hotter and put a lot more stress on oil than something that is liquid cooled. Many also don’t include a filter and many are often neglected by the owner who seems to overlook the similarities to their car.

I hear arguments about if it is ok to run diesel spec oil in a gasoline car engine because of the emissions equipment such as O2 sensors, cat converter, and such. I also hear that modern diesel oil is much lower in the additives that foul such equipment as diesel engines are now being more controlled for their emissions output.

Many of the ACEA European spec oils for BMW, Mercedes, Audi, etc. are also rated for certain diesel engines. I understand this is really good stuff. I run this in my little Geo Metro as these engines work hard and the lifters are pretty picky. All those on the forums for these little cars suggest these types of oil for them. I am sure that they would also do just fine in a mower, especially if you get the proper weight.

When I was in 5th grade, I changed the oil on our family lawnmower and put in 10 weight. The mower really got hot. I thought oil was oil. My dad discovered what I had done and put in the correct 30 weight oil. The engine was a 4 stroke Lauson engine (I think Tecumseh bought this company). Apparently, no harm was done.
Just yesterday, we returned home from an out of town trip and I needed to mow the yard. The oil was a little low on the dipstick of my mower’s Briggs and Stratton engine, so I topped it off. I was out of straight 30 weight, so I topped it off with 5W-30 that I had opened. I did get some blue smoke that hadn’t happened before and I don’t know if it was the lighter oil that I used to top off the crankcase, I slightly overfilled the mower, or the grass was really thick and the engine was pulling hard. The mower is 21 years old, although I put in a short block about 9 years ago.
I think a straight 30 weight is best. Tom McCahill, who wrote for Mechanix Illustrated magazine years ago said this about multi-viscosity oils: “Multiweight oils are sucker juice. 10W - 30 is a lousy #10 and a lousy #30”. I followed his advice in the cars I owned and used 30 weight in the summer, 20 weight in the fall and spring and 10 weight in the winter. The cars I owned at the time did just fine, but when I purchased an almost new 1965 Rambler back in 1965, I followed the recommendation in the manual and did use 10W-30.
However, an air cooled gasoline engine is different than a modern car engine. These engines have a carburetor instead of fuel injection and hence the oil is more apt to be diluted with gasoline. I use a straight 30 weight oil in my tiller and lawnmower engines. My generator says to use 30 weight in the summer and a lighter weight in the winter, or to use 5W-30 synthetic oil year round. Since I don’t know when we will have a power failure, I do use the multiweight synthetic oil in the generator. However, I only use the mower in the summertime, so I follow the recommendation and use straight 30 weight heavy detergent oil.

For my '02 Chevy Silverado 15, with the 4.8-L v-8, the engine specs recommend 5w-30 oil, which I have never used in any vehicle I’ve owned. Here in the warm, sunny state of Florida, I use 10w-30 full-synthetic oil year-round, and have had no issues so far. Today’s cars do fine on multi-viscosity detergent oils, but I think synthetic is the best, even for small power equipment engines.

Today's cars do fine on multi-viscosity detergent oils, but I think synthetic is the best, even for small power equipment engines.

It’s a no brainer for small power equipment engines to use full synthetic. Most home owners just change the oil once or maybe twice a year. The added cost is MINIMAL.

@Mike <blockquote=MikeInNH>Most home owners just change the oil once or maybe twice a year. The added cost is MINIMAL.

The last time I changed mower oil is like 50 mows @1~15minutes/mow, ~130yds/mow. IOW, NEVER. I hate mowing and just assume that the mower would die like my grass.

Might as well hijack the thread and turn it into a small engine discussion. I just serviced the mowers and snow blower at the cabin last weekend. I’m pretty careful about the oil I put in but have to sympathize on the carb gumming. BIL couldn’t get the blower started even though I had put a new carb kit in last fall and run the fuel out of the carb each time. The nozzel was all gummed up again. Cleaned it out and worked fine. I try to emphasize using B&S fuel stabilizer just for this problem. Not all my relatives listen to me though.

My mower and snow-blower have fuel shut-offs. Before I put the blower or mower away for the season I start it up…then shut off the fuel flow…and let it run until it stops. Never had a problem with carbs gumming up.

out here in the wet part of the PNW, we also mow in the winter. :>((
But the wettest part (coastal & sandy) I mow only in March-May.

@Rod_Knox, why would you use a special oil for storage? If you change the oil using 30 weight oil in the fall when you put the mower into storage, I don’t see why it would be necessary to change the oil again in the spring.

Yeah, it would be one thing to use a 5W30 instead of 30W for storage but think you went too thin. Why use a storage oil that is different than what you normally run? I could see running 10W30 or 10W40 year round.

Yes, synthetic is great for small engines as they really put a lot more stress on oil than a normal water-cooled and fuel injected engine. The fact that these engines are often neglected and used in a very dirty/dusty environment makes oil quality and frequent changing important.