"I have the feeling they'll say no"
Don't know until you try. Off to the dealer you go. The dealer will do an engine failure analysis and will know if it was caused by a defect or lack of lube. You may want to document your break in oil change and how much oil and when you've topped it off.

Was that a B&S engine or Tecumseh? Just curious. Actually, come to think of it, I guess Tecumseh engines are no more. That doesn't sound typical for either engine though. You might have simply had a mechanical failure that they (Toro) will back you up on since it is new. Good luck!

'I have the feeling they'll say no'

Not so - most mowers and certainly a major like Toro have at least a one-year warranty, and the main thing it covers is catastrophic engine failure as long as there was oil of the correct type and amount in it. Just take it to any Toro dealer - they can run the serial number and tell when it was made, and you probably have a receipt that will tell when it was purchased. This kind of failure is rather unexpected and usually is oil-related, so be firm in relating your oil history with it. If you have some of the oil you've been using, take it with you to prove proper type/viscosity. One thing that bothers me is that you had to add oil, which implies the engine had already been using oil. This is also strange for only six months of use. This begins to add up to what looks like a case of running on low oil at some point, when the original damage was done that led to the eventual failure. Since Toro can neither prove nor disprove this, most likely they will honor the warranty in favor of good customer relations. Using the mower extensively on extreme slopes could result in lubrication failure.

Saxman makes some very good points. Please come back and let us know how Toro handles your claim. It can help others here when they decide which brand to go with. I am guessing if everything you say is fact that you will soon have a new mower.