[]"So, is the straight 30 wt that comes with the mower good stuff? It's Briggs & Stratton branded oil or some unknown name. Or should I avoid that and use a good brand like Pennzoil or Valvoline?"[/i]
OEM branded oils are at least OK but rarely "special" like they would have you believe. Buy a name brand oil like Shell, Pennzoil, Chevron Delo, etc ... instead.
The rub on XW-30 oils is that they'll easily shear down a full grade before they are drained by most people and when the oil is that thin, it can get consumed quickly. If the unit is run out of oil, it will likely (but not always) seize.
If you check the oil every 10 hours or so and change it once per year, the motor will outlast the rest of the unit.