its ok to use multi in your lawnmower. the reason its not suggested is because most mowers use splash oiling. well tecumsehs have a little oil pump that runs off the cam and pumps oil upto the top main bearing and to the top cam bearing as well. briggs use only splash oiling as best i remember. 10w-30, 20w-30, 5w-30, as long as its something 30 you're alright. mowers arent super sensitive to oil viscosity changes.
My mower is a Sears Craftsman with a 4.5 hp Briggs & Stratton engine. I kind of suspected that it wouldn't be particularly sensitive to viscosity, as long as the max was 30W. I assume that once it's warmed up, the oil will be at 30W and should perform more or less normally. Maybe the lower viscosity when cold might permit it to be more completely lubed while it is coming up to maximum operating temperature.
I will do as another reply suggested and make sure the oil is well mixed, though I will do this by carefully rolling and inverting the bottle rather than shaking it, as to avoid filling it with air bubbles, even though they may not do any harm. I guess the oil gets splashed around pretty well in the engine anyway.
Now the remaining question is this -- does going from a dino oil to a full synthetic mean I can extend my oil change interval? It is nominally 25 hours, but due to the small size of my yard, I doubt I run that engine for even half that time each mowing season. I wonder if there's any way to put a Hobbs meter on an engine that size? Without any sort of a DC circuit, I don't know how you'd do it, even if it were economically feasible. I've seen professional riding-type mowers that had them, but they had a DC circuit for the ignition, electric start, and lighting, so they could wire it to the hot side of the ignition switch. I guess I'll just do as usual, and keep changing the oil each mowing season. I'll most likely make the change next weekend, if it ever stops raining...otherwise, I'll have to mow the whole lawn with my Poulan Pro string trimmer! I've already done that to fairly large, fast growing sections this weekend.