I bought a can of Pump Saver in 2002 when I bought a new pressure washer. Every fall I spray this into the water inlet, which forces the water out of the pump and leaves an oily preservative. Six years later my pressure washer is like new, and the can of Pump Saver is still going strong. Hopefully I'll get another season or two out of it. Use it anytime you plan to go 30 days or more without using your pressure washer, especially if you're facing freezing temps.
Blew up on me and made a mess
Have used for years on professional pressure washers with great results. Use every fall / winter on machines left in storage sheds and freezing temperatures. Always good results, no problems. GOOD STUFF.