Actually, it's a complete waste of a dwindling resource. Changing your oil more frequently than necessary is environmentally irresponsible and provides NO benefits to your engine.
As I said before, your car came with a Maintenance Log - or at least it should have. Check the documentation package that came with the car.
Waste of money, IMO. Additionally, aftermarket warranty companies have a disturbing habit of going out of business. If the extended warranty isn't from the factory, you can't count on it being there when you need it.