Thanks for reporting back Glenn - a thread that ends up with no conclusion is frustrating for future readers.
For the usual range of daily temperatures Briggs recommends a straight SAE 30 oil (not multigrade) with an automotive rating, such as SJ etc. If you use multigrade it will burn more oil, though it won't actually do any harm.
When you drain the oil, get the engine warm first, then tilt the mower steeply toward the drain (usually the filler hole, but there is a drain plug underneath the engine as well) so that the drain is the lowest point on the engine. This is so that as much of the accumulated sludge comes out as possible. Leave it tilted and draining for half an hour or so. Clean and gap the spark plug, and test the ignition by pulling the starter with the spark plug clamped to a metal part of the engine - expect a steady series of blue sparks. It is useful to check the tappet clearances, with an unfamiliar engine - behind the valve chest cover, near the crankcase. Check that the cooling fins are clear of grass and debris, both on the cylinder and the cylinder head. If it is running well without smoke and there is no oil around the muffler, I personally would not pursue it further than that.
