[jayhaitch]

I remember reading somewhere (but can't remember exactly where....) that a petroleum 10W40 turns into a 10W30 in a relatively short time by breakdown of these viscosity index improvers. I seem to remember something like 3 or 4,000 miles in a car. Not sure how this would translate into hours on a pressure washer.

That's why old outdoor power products specified straight 30 weight. No VI improvers to break down, less sludge, less oil passage blockage, assuming the engine is used in warm weather only.