Mick, the "huffing and puffing" I was asking about, was the extent of gas blowing out of the oil filler when the dipstick was absent. The engine should not run much differently with the crankcase open: on cars, the idle speed used to pick up a bit in the old days before automatic idle speed adjustment, but you'd never notice such small effects on a mower engine.
From your description, there isn't a lot of blow-by happening, since if there were, you'd see something like a thin sphere of oil mist around the open end of the filler tube. If you've ever seen a tired old car engine idling with the filler cap removed, you'll know what I mean. Of course the amount of gas expelled from the filler increases exponentially when the engine is under load. Those of us old enough to remember the "road draft tubes" running downward from car engines prior to Positive Crankcase Ventilation, and the vast cloud-banks that used to roll out from under old cars when they attempted to go uphill, will never forget the image.
It sounds as if the only issue is that there is a mismatch between your expectations, and your engine's design and manufacturing standards.
