[PNW Fireguy]

This is a sad tactic used by far too many companies today. The Magnuson Moss Warranty Act of 1975 was passed by the US Congress to prevent this companies from doing this. While they are free to make these types of recommendations they may just be directed to folks fears. The act specifically precludes manufacturer's from denying or invalidating a warranty soley due to use of an add-on product unless the manufacturer can specifically prove that the add-on product caused the damage.