When I had an F-150 half-ton it used AMSOIL 100% synthetic engine oil with twenty-five thousand miles "under it belt". That used oil is what I put into my lawn mower. When the mower needed new oil I took that used AMSOIL synthetic to a Canadian Tire store that let me dump it into their used oil...
Back in the day when I had small equipment like that, ie. rototiller, etc. I would put the used AMSOIL from my half-ton in them but when AMSOIL's "Small engine synthetic" became available it was a no-brainer.
Had a similar issue with our snowblower, ie. gasoline leaked from the tank into the oil pan. Was told to make sure the gas tank is drained before being stored for the summer.
Concur with replies about ethanol and stabilizers but I would also add the suggestion of using quality synthetic engine oil that has and is the S.A.E. viscosity the owner's manual shows.
When I mowed my lawn with a Lawn Boy a few years back spraying the whole deck while it was still new with the AMSOIL product, M.P. Now while using a snow blower I spray this product onto the housing and blades so that the snow does not stick.
My suggestion is "Saber Professional Synthetic 2-Stroke Oil" as shown in the AMSOIL Factory-Direct Catalog.
Rudy Hiebert
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Just followed the email to this forum; thanks for taking up the cause of better lawns, etc. It's a cool place for connecting. I'm here in west-coast of Canada were the growing season has been in full swing for a month but could still use a bit more sunshine. I'll try to answer any questions...