[Lynn Eades]

Keiths motor has good spark. Usually when the flywheel key is damaged the spark drops off to a point where it won't fire, so it's probably not the key/timing.
Bob, the later Brigg's have a solid state ign. no points- much more reliable.
On the oil thing, As you say with the Briggs motors a mono grade oil is required. So too do the Ford V8 flat head motors,require a mono grade oil. ( I know it, but cannot remember why. I think it is to do with heat transfer)