I haven't found a way to change the oil Rob - it's described as "maintenance free", and I don't think you can access the oil without removing the engine then stripping the pump. Older pump designs, and the continuous-service commercial units which usually have belt drive, have regular oil changes. Here is a description of how to do it:

http://www.briggsandstratton.com/us.../pressure-washers/water-pump-maintenance

The really old designs of pumps, once popular in the US, had crankshafts and two cylinders. Those ones are intended to be overhauled, and require oil changes every 50 hours. The more modern consumer-type designs seem to have 3 cylinder swash-plate pumps, and those ones typically have no external access to the oil.