[AT] B&S engine oil

[Larry Goss]

That IS the way it works, Charlie. Multi-viscosity oil is basically a light weight oil that has additives in it to keep it from thinning out at the operating temperature of the engine.  The problem is that air-cooled engines have a tendency to run hotter than water-cooled ones, and the additives boil off or burn out and leave you with nothing but the basic light-weight oil.  Before you know it, you get piston or bearing galling because of metal-to-metal contact and the engine is ruined.  It doesn't take much to make a multi-viscosity oil work like it's supposed to even in an air-cooled engine.  Pressurized oiling, an oil filter, and a little larger sump (so there is more oil to dissipate the heat) are often enough to lower the oil temperature so it keeps the additives instead of burning them off. Some engine brands make use of an optional oil cooler to keep the oil temperature even lower.  Pressurized oiling systems are so commonplace today that "putting the wrong oil" in a small gas engine isn't nearly the problem that it used to be, but those of us who work with older splash-oiling engines still have to be aware of the problem and act accordingly.

Larry