Air cooled engines need the very best oil?

Joined
May 7, 2018
Messages
818
Location
Northern KY
Does it make sense to run a really high quality oil in an air cooled engine? I would think in summer an air cooled engine gets hotter than the water cooled equivalent and would put more stress on the oil.
 
Joined
Apr 15, 2010
Messages
8,044
Location
Connecticut
No. There are a lot of myths floating around about small engine oil and what they need. While small engines do run hotter, it isn't necessarily that much hotter than the oil temp on your car. Modern passenger car oils are very good at standing up to the heat, since they are designed to withstand abuse from direct injection, turbo chargers, complex variable valve timing mechanisms, etc.

Another myth is that they need high ZDDP. Also not true. Most small engines do have a flat tappet cam, but the valve spring pressures are extremely low, so low that you can open the valves by pushing on them by hand in most cases. The most important thing with a small engine (especially splash lubed with no filter) is to keep the oil level full, and change it at the correct interval to get out the contaminants. Those are the most important factors. One might even bet that if you run a cheap oil but change it sooner you would be better off than running an expensive oil.
 
Joined
Sep 23, 2008
Messages
10,938
Location
Ontario, Canada
Yeah I doubt many ever get wore out prematurely from the wrong oil, most die from having not enough oil, or super long OCI's... The ~50 yr old briggs 3.5 on my grandfather old front tine roto tiller gets whatever is handy on the heavier side, but it has finished off a quart of 0W20 without complaints...
My air/oil cooled atv gets decent HDEO though, but its got a common sump for the transmission and it kind of expensive to replace!
 
Joined
Dec 4, 2013
Messages
788
Location
Maryland
Does it make sense to run a really high quality oil in an air cooled engine? I would think in summer an air cooled engine gets hotter than the water cooled equivalent and would put more stress on the oil.

Your Thread Title is a bit of a myth, it is more important to make sure the oil level is correct before you use the mower.

I would get yourself an infrared gun, and with the engine running whether it is a push mower or a riding mower, shut the mower off and take the oil fill cap off and aim it into the engine. See what your oil temps are.

I have seen temps between 205 and 220 during the summer and during the late fall when it is cooler, around 190 to 200.

I did the same thing with a snowblower after using it for like 2 hours in temps that were about 10 degrees out, the oil temp was like 176

As long as you have proper airflow and no obstructions with your air cooled engine, then you do not have to have the best oil. If you have the best oil and an air obstruction with your engine, the best oil will not help.
 
Joined
Jun 4, 2003
Messages
1,642
Location
98245
... The most important thing with a small engine (especially splash lubed with no filter) is to keep the oil level full, and change it at the correct interval to get out the contaminants. Those are the most important factors. One might even bet that if you run a cheap oil but change it sooner you would be better off than running an expensive oil.
Exactly. Two things to keep in mind about air cooled engines:
1. They operate over a wider temperature range, which means bigger tolerances & clearances.
2. Because of (1) they contaminate oil faster than water cooled engines do.

Typically, air cooled engines will contaminate oil before they stress or shear it. With air cooled aircraft piston engines (Lycoming & Continental) the limiting factor for oil change interval is contamination, so the oil change interval is twice as long (from 25 hours to 50) when you use a filter. Of course they also run leaded gasoline which increases the contamination, but even with unleaded gas an air cooled engine still has rapid contamination due to (1) above.

Furthermore, mineral oil is actually better than synthetic at keeping contaminants in suspension.

All that is to give some reasons why cheap mineral oil changed frequently is the best approach for air cooled engines.
 
Joined
Nov 23, 2015
Messages
1,967
Location
US
Somehow or another within my wife's family I've become "the guy" for lawnmower maintenance, and I've changed more lawnmower oil and sharpened more blades this year than I probably had in my life up to this point.

Generally they get whatever I have an open quart of, although if I have to buy oil it will be conventional 5W-30 or 10W-30. I've gone as high as 20W-50 before because, well, I have a British car and even if I have no other oil on hand, I'll always have 20W-50. Generally during mowing season it's warm enough here that I don't worry too much about 20W-50 as long as it's not too hard to pull.

Some seem utterly alarmed at what seems a "reckless" disregard for the oil type I'm putting in, but clean and full trumps basically any other consideration.

Which reminds me too-BIL is supposed to bring his tomorrow for a sharpen and oil change. I've definitely gotten good at freehanding them on the bench grinder!

And I'm so grateful too for my own personal push mower(s) where I never have to worry about changing the oil or even checking the oil level :) (plus I cycle through mixed gas a lot faster now than I ever did in the past).
 
Joined
Jun 5, 2003
Messages
25,991
Location
Apple Valley, California
Air cooled dirt bikes and such ran decades with little or no maintenance and no oil filter.

My dad owned air cooled vw's. Both of them ran well over 100 k using Pennzoil 10w-40 with pz-7.
 

4WD

$50 site donor 2023
Joined
Sep 21, 2010
Messages
20,807
Location
Texas via IAH
My lawn tractor gets lots of leftover oil - out of that? - HDEO is the cheapest and works well …
I’m more likely to spend a bit more on my generator - but not like it holds that much anyway …
 
Joined
May 17, 2006
Messages
4,502
Location
Central Wisconsin
Typically, air cooled engines will contaminate oil before they stress or shear it. With air cooled aircraft piston engines (Lycoming & Continental) the limiting factor for oil change interval is contamination, so the oil change interval is twice as long (from 25 hours to 50) when you use a filter. Of course they also run leaded gasoline which increases the contamination, but even with unleaded gas an air cooled engine still has rapid contamination due to (1) above.

A couple of differences however. Air cooled aircraft engines all have oil coolers, and they operate for most of their running time, at altitude where
the air temperature is lower than ground level.
Many use synthetic oil in OPE/lawn equipment with good results. Also, some have checked oil temps after a hour or so of running and found
temps in the 230/245 range. That is the temps where conventional oil starts breaking down.

My 2¢
 
Joined
Jan 22, 2011
Messages
7,911
Does it make sense to run a really high quality oil in an air cooled engine? I would think in summer an air cooled engine gets hotter than the water cooled equivalent and would put more stress on the oil.
Small engines (air cooled or not) are the most forgiving pieces on Earth. You could put the very best or the cheapest oil in and they keep going with no damage either way. THE KEY IS TO CHANGE THE OIL WHEN YOU ARE SUPPOSED TO.
I have always used house brand conventional oil in all my air cooled engines. Some of the oldest ones still in use were made in 1972, 1984, 1989, 2008.
Too many overkill with the oil and gain nothing.
 
Last edited:
Joined
Oct 23, 2017
Messages
1,030
Location
VA
Does it make sense to run a really high quality oil in an air cooled engine? I would think in summer an air cooled engine gets hotter than the water cooled equivalent and would put more stress on the oil.
Guess I'll assume we're talking about mower/tiller etc. If that is the case, those engines don't run nearly as hot as people might think. Those things have a huge metal surface area to dissipate heat that gets forced air blown across it all the time. Most newer/high(ish) end motors like this also have an external oil cooler of various designs.
 
Joined
Sep 10, 2005
Messages
1,367
Location
Erie, PA
This is my opinion and I have torn down a lot of small engines. Because the smaller ones do not have an oil filter, it is more important to change the oil every now and then to get all of the metal wear particles out of the oil. My first oil change on my power washer was alarming and full of aluminum glitter.

It is more important to use a standard oil and just change more often than selecting a synthetic and having to keep it in longer to recoup your dollars spent.

Cylinder wear I find is mostly caused by air filteration issues, and crankshaft journal wear is usually caused by low oil level (upper journal) or by extended oil drains in which the wear metals eat the crank pin and rod surfaces.
 
Joined
Jun 4, 2003
Messages
1,642
Location
98245
A couple of differences however. Air cooled aircraft engines all have oil coolers, and they operate for most of their running time, at altitude where
the air temperature is lower than ground level.
Many use synthetic oil in OPE/lawn equipment with good results. Also, some have checked oil temps after a hour or so of running and found
temps in the 230/245 range. That is the temps where conventional oil starts breaking down.
My 2¢
On climb, aircraft piston engines run at full power with less cooling due to the slower airspeed and the pitch angle of the cowl. Cylinder head temps can get to 400* F or so. They may do this continuously for 20-30 minutes to reach cruise altitude, where they'll be operated typically around 70% of rated power.
 

wwillson

Staff member
Joined
Aug 20, 2003
Messages
5,359
Location
Naperville, IL
On climb, aircraft piston engines run at full power with less cooling due to the slower airspeed and the pitch angle of the cowl. Cylinder head temps can get to 400* F or so. They may do this continuously for 20-30 minutes to reach cruise altitude, where they'll be operated typically around 70% of rated power.
Which is why you transition from best angle to best rate of climb as soon as practical.
 
Joined
Jun 4, 2003
Messages
1,642
Location
98245
Which is why you transition from best angle to best rate of climb as soon as practical.
True. And you can climb at an even slower rate / flatter pitch angle / higher airspeed for additional cooling, where terrain permits. The training says use a rich mixture to improve cooling during the climb, but lowering the nose / pitch angle is even more effective than that.

PS: lowering the nose is also good for visibility & safety. In many airplanes, even Vy (let alone Vx) has a steep enough pitch angle to reduce forward visibility.
 
Top