Keep an eye on it. Maybe switch to -30 and see if the oil level actually increases as it picks up moisture in local driving. Or continue to use -20 if there is a chance of 600 miles of longer, higher speed trips. If the consumption is > 1.1qt after 600 miles then think about a powertrain warranty claim.
Yeah, this really sucks. Older Toyotas didn't suffer this kind of oil consumption. I'm not sure if the problem is with the thin oil or cost cutting in engineering.
I hope your extended warranty is with Toyota and not some aftermarket plan ......... if the latter, you will probably never see a dime in reimbursement if something goes wrong.
First, I hope you kept all your receipts for oil and filters. If you bought your oil in bulk, I hope you kept a contemporaneous log. The fact you did the work yourself not be a problem. Here is an example of a failed engine in a 4Runner under warranty in the Bobistheoil guy forums where the engine blew a rod despite regular changes where the guy did his oil changes but bought the oil in bulk for many cars but he did keep a log of oil changes.
When the Toyota dealer pulled the engine apart, they could see the inside looked like brand new, oil was not the problem and the guy got a new engine under warranty.
Here is the rub for you ........... if you switch to 5W30 oil ...... depending on the dealer ......... they may say any future engine damage is due to using other than the recommended grade which could present a hassle.
I suggest you go the dealer and document your issue now so if it comes up later, they have some record of it.
Note: Last January, my Ex Wife bought a 2011 Highland V6 that came with 0W20 synthetic and requires 10,000 mile /one year oil changes. She has the same 2GR-FE engine I do in my 09 Camry V6 which requires non synthetic 5W30. It uses ZERO oil between 5K oil changes. Her car requires a 5,000 service - which is free for two years - but they just check fluids, tire pressure, etc. [no 5K oil change]. When she brought it at 5K miles after a 2000 mile trip from KS to Colorado and all over the place, the dealer did not have to add any oil. She never even checked the oil during the first 5,000 miles since our family has been driving Lexus/Toyota vehicles for over 20 years and not a one of them has ever used ANY oil. Surely if her Highlander had even used a half quart, they would have either topped it off or told her to be sure and check it before 10,000 miles. Thus, the 20W oil probably makes minimal difference in oil consumption in most Toyota vehicles.
Again, good luck.