My Craftsman mower with a GCV160 will be 10 years old next month (manufacture date of 7/2003). For oil, it gets whatever is left over from the cars or a motorcycle. I figure since it does not have an oil filter, having clean oil is probably more important than exactly what is in it. So, it get changed a few times a year as I always have bits of left over oil:
10w-30 (any "name brand" dino on sale)
10w-40 motorcycle oil
and sometimes a 50/50 mix of 5w-20 ("name brand" dino on sale) plus Rotella T 15w-40.
After each mowing, I turn off the fuel valve and let it run until it starts surging, then shut it off. At the end of the season, I turn off the fuel valve and let it run until it dies.
After almost 10 years, it still usually starts on the first pull.
At this point, the only thing that I can see that might keep it from lasting as many years as my two cycle Lawn Boy would be the steel deck, versus the aluminum deck of the Lawn Boy.