'Overwintering' petrol lawnmowers

I can think of several. Manufacurers such as B&S usually recommend an SAE 30 mineral oil. These have an expected life of about 6 months before the carriers in it start to braeak down.

Then, when the engine is running, acids and other contaminents that can damage the engine build up.

Even if very few do it, the best thing is to warm the engine at the end of the season, remove the oil, and refill. In Spring, start the engine, get it warm, remove the oil, and replace it with fresh.

 

All my mowers are emptied in the spring when serviced and fresh oil added then. The workshops normally strip and clean carbs, replace pull string and check pulleys, blades, drive gear and safety mechs including rpm. I normally run the petrol out so it’s empty but the Honda engines I have include a inline valve and I’ve added petrol filters to my Hayter which is a tad prone to crap entering the tank.
My local Etesia dealership has my 2006 model looking like new each year and while not cheap it’s been faultless.

 

I filled my Briggs Stratton powered mower with Shell Helix 10W/40 back in May 2014 and I have never felt the need to replace it. It is still spotlessly clean, not gritty between the fingers and still up to the full mark. I do not drain the petrol from it. First start of the new season I remove the spark plug (still the original), crank it over a couple of times to make sure the bore has a bit of oil, refit the plug, prime the carb and it starts first time. It actually starts better then than it does on a warm summer's day when sometimes it takes two or three pulls of the cord.
I have washed and reoiled the air filter once and I use E5 97 RON petrol.