A higher viscosity may reduce mpg at longer intervals. Okay, whatever that means.
Toyota has yellow warning flags all over the manual but none that says a higher viscosity may damage your engine. So this means nothing to you "Your vehicle is certified with Genuine Toyota 0W-16 motor oil. For VEHICLES CERTIFIED FOR 0W-16 motor oil, the oil change interval is 10,000 miles or 12 months IF 0W-16 MOTOR OIL IS BEING USED. If 0W-16 is not available, 0W-20 mineral oil may be used. However, it must be replaced at a 5,000-miles or six-month INTERVAL with 0W-16 motor oil"
Bottom line is if you live in a higher climate area use increase viscosity in cooler use 16. Our OMs must be different. I don't see where mine specifies this.