Thanks for the further input. From the age an mileage, I surmised you did a lot of short trip stop and go driving. At 20,000 miles per year in North Texas, you are changing at the right interval, but you still get the best value from using a standard mineral oil and changing it at 7500 miles.
Synthetic is not “bad” for your car, but it is EXTREMELY slippery and gets through tiny openings that normal mineral oil doet not penetrate. So, I predict that when you switch to synthetic your car will “use” more oil, but it will leak out through places you never suspected. However, do the experiment, but don’t go 15,000 miles between changes. Those drain intervals are used to sell the oil and justify the higher price. If the car was new you would be voiding the Toyota engine warranty.
Regardless of the Castrol claim on the bottle, I would not exceed the Toyota drain interval of 7500 miles. Does Castrol also recommend a 20,000 mile oil FILTER?
I use Mobil 1 synthetic in our Nissan Sentra, which has a 4000 miles drain interval for severe service, which this car is used for (short trips). I drain it twice a year at about 4000 miles and change the filter. We live in a cold area, and do short trips with this car.