Pete, I do not dispute that light oils have lower viscosity than heavier oils. That does make it easier to pull the starter on a cold engine, and it also makes it easier for the oil to get past the piston rings. B&S authorise the use of 5W30 in their engines, so you are not contravening their instructions by using it. You will certainly use more oil, and you may incur a higher wear rate under severe operating conditions. On the positive side, you will reduce the frictional horsepower whenever the engine is not running hot, and therefore save a small amount of fuel. For what it is worth, Honda recommends the use of a multigrade oil in their mower engines, and Briggs do not. Either Honda is more confident about their piston rings, or they are more mindful of how difficult it is to get monograde.
If it significantly helps your mother to cold-start her mower, and that otherwise would have been difficult for her, I can well understand you using the light oil in her mower.
