If you read the fine print, you'll find that the guarantee doesn't even really cover anything. If your engine dies from overheating or something else not related to the oil, they won't cover it. If all the oil leaks out somehow and the engine siezes, it's not covered. If you never change the oil, it's not covered. And it doesn't cover normal wear and tear (worn rings, valve seals and guides, etc) Soo....what else is there? There is no other way an engine can possibly fail!
It's pretty genius really: Make a warranty that doesn't actually cover anything, and people think they're getting a really great insurance policy and buy tons of product. Kinda wish I'd thought of it.