This is what Briggs and Stratton oil recommendation says:
Vanguard 15W-50 - Varying temperature range. For continuous-use, such as commercial lawn cutting or pressure washing
Houston at 98-99 degrees? A 50 weight is IDEAL.
Pressure washers are high demand, high load. They heat up the oil more than your garden variety mower. Only a commercial mower running non stop in heavy wet grass would be similar.
I had a small engine shop and Briggs added the 50 weight oil recommendation because of too many blown up engines on commercial equipment. They only recommended 30 weight for a decade or two. They modified their recommendation. Why? Real world evidence.
This home owner pressure washer with the sealed pump is a low hour use unit. He probably will use it for 5-10 hours per year. Realistically ANY oil used will be fine, as long as it’s kept full and not allowed to get low. It’s a Honda clone engine and Honda has only ever recommended basic conventional 10W30 oil in these style engines for decades.
The sealed pump is only rated for 50-75 hours before it grenades. At which point he buys a $70 replacement and slaps it on, and good for another 7-8 years.
Starting can be problematic with thicker oil. You have to fight the viscosity from the oil and the pump building pressure and compressing water. For that reason alone, I use synthetic 10W30 or 5W40 oils in MY pressure washers in south Florida. I meticulously maintain my equipment and the oil will never be low. If I was using it daily commercially, I would run Mobil 1 15W50.