You said, my new Tundra. You are correct the Toyota engine stays tight for a good while. I have seen GM engines break in at 3000 miles due to loose tolerances.

I had a '03 Corolla with the 4-cyl engine. I was anxious to get it over on full synthetic oil. I decided to upgrade to synthetic oil at 3000 miles. That was a mistake as the gas mileage increase was little. I saw the gas mileage jump up at 8000 miles and jump again at 8500 miles. I thought that engine would never loosen up. My fault putting the synthetic oil in too soon.

It was nice once it was broke in. I started getting 39 and 40 mpg consistently. My best was 42 combined here around town. We live 8 miles south of town so that includes some slow highway miles.

My advise with your 13.5 mpg is wait it out. Might go back with the same weight dino oil till the second oil change then consider synthetic. Could be that 5000 miles you could see a increase in your gas mileage. Compare that to your trucks mpg sticker. Be sure to use a quality brand of gas.