These days ethanol is likely the greatest cause for premature failure of small air cooled engines. But then there’s the decompression mechanism that causes broken starter drives and electric starters. How many lawn mower owners open that manual and follow the instructions to regularly adjust the valves? But I’ve never kept a lawn mower running 20 years @dagosa. You may hold the record there.
The only one I’ve ever had fail is on my snow blower. Seized up starting to clear about 10" of snow and that’s all she wrote. It was about 30 years old though that I paid $50 for used. When I tore it apart it looked like it was the crank bearings that went. It was in January in Minnesota after a hard winter. I only found three snow blowers left in the Minneapolis/St. Paul area. One was waiting for parts so pickens were slim. I had to either come home with a new blower or a new engine.
I hear you. Ethanol has been the biggest problem since non ethanol gasoline has made made an appearance in this area. My last two self propelled mowers have gone two decades till drive train failure…they have then been turned over to my son whose a flat lander. Carb trouble then turned out to be the biggest culprit for him. At 20 years, the biggest problem has been finding parts. Still, never had an engine failure due to lubrication.
A big reason for longevity imo, has been regular oil changes and keeping the blade sharp.
Yup, like I’ve said umpteen times before, once I switched to the non-oxy fuel with B&S stabilizer year round, I haven’t had a bit of carb problems. It’s available here as premium for classic cars, boats, etc. A little more expensive but before that I had gotten one week on my mower fuel pump before it went again.
No doubt, @dagosa. just simple basic maintenance has for years resulted in air cooled engines on yard equipment running until the handles rusted off. I have my doubts about the current crop of air cooled internal combustion engines though.
NNLike you say, It’s important imo, to be very fastidious with the simple things, like keeping the deck clean and the blade very sharp. I’ll sharpen mine several times during the cutting season. I’m on just the the second mower in 22 years of grass cutting on our lot which is very steep. IMO, air cooled lawn mower engines don’t like the extra drag that from clogged decks and dull blades. That’s just an opinion from the golf course maintenance personell I knew over the years.
Steep lots can shorten the lives of small 4 stroke engines over the years if you’re no careful about spending too much time mowing in one direction. The oil just doesn’t get pumped out of the sump as well according to maintenance people. Two stroke mowers had a big advantage in hilly areas.
That’s probably the single most important thing you can do for your mower. My generators were literally inoperable if I didn’t drain them after every use and run tru fuel through them. I hated using leaded aviation fuel but I had no choice. Now it’s all propane in them and driving the twenty miles to get ethanol free gas for the outboard engines and all the two stroke cutting tools…trimmer and chain saw and hedger.
I have a 1989 Simplicity tractor w/Briggs engine. It works perfectly. This year I decided to paint the underside of the mower deck with POR15 to address rust formation. I have done maybe two oil changes over that time. I just add a bit of oil mid season to top it off. I have never changed/added any oil to the hydrostatic drive unit but look to see it’s full at the start of the season. It cut an acre a week up until 6 years ago and now cuts two+ acres. This is the most abused piece of equipment I have ever had and it hasn’t had one issue in all that time other than needing a new battery every 4 years or so. At one point I was afraid to mess with it, thinking it will get soft and expect good treatment 
then it became a challenge to see how long it would run. It’s probably going to win as I’m interested in getting a new tractor with a larger deck to reduce the time required to mow.
My first lawnmower was a $29 KMart special. bought in 1968 when I bought the house. It was a 3 hp 22 " cut, very light and easy to push. It died after 22 years when one of the bolts and washers that were holding the cracked deck together failed and the blade hit a curb and bent the shaft. I changed to a small 1978, 32" International Cub riding mower in 1990 and used it until 2015 when a pulley failed and I couldn’t get a replacement.
Our churches’ 1995 John Deere 325 cuts our 8 acre lot at 23 years old and doesn’t burn a drop of oil. All of these machines had their oil changed at 25 hours except for the Deere which calls for every 100 hours. The hydro fluid on it gets changed at 500 hours.