"I have in the past run the old 1993 8HP Cast Iron Sleeve Briggs for 7 days with oil changes every 20-25 hours of run time"
OMG, seriously? You change oil every 2 days?
I have an 11hp honda in a pressure washer that gets probably 500 hrs a year use. It gets USED. Sidewalks, carpet, the siding on the house, the roof, parts, commercial mower, swimming pool, deck, bbq, compact track loader, 18-wheeler and my vehicles are proof: no rust.
I think I've changed oil in it 3 times in the last 17, 18 years?
What do I put in it? Pfff, whatever is laying around, 15-40 diesel-spec.
Still starts on the first pull with the choke on.
Oil changes are over-rated: up to 10,000 mile intervals is not un-common. And, NO, it's not "cheap insurance", I have yet to have anything mechanical let me down because the oil was "old" or "houred out". It's a waste of money if you fuss around changing oil because you seen a pennzoil commercial on tv