Royal Purple 5w-30w at 23,500 miles.

Just switched from Valvoline Synthetic to the R.P. in the '05 Double Cab with rear wheel drive. The Toyota techs put 35 psi in all the tires as per
their standard operating procedure. It had been running the 29 pounds
per the mfg's recommended pressure and the mileage was lousy. Hopefully
this will make a big difference. I will be interested to see what kind of products will make a difference in fuel mileage improvements, as well as any extended idle speed operation during emergency conditions when the truck is used as a backup generator.

 

35-40 psi will improve both mileage and tire life.
You have a Prerunner... moving down to a highway tread tire or at least a less aggressive LT tire will also help. Offroad tires are not that good for mileage.

Synthetic oil will not make a difference in MPG, but it will make a difference in your engine's life. Just stick with the factory recommended change intervals at 5000 miles, and use OEM Toyota oil and air filters.
Avoid K&N air filters, they will allow dirt to pass through that will undo all of the engine "saving" you are doing with synthetic oil.

IMHO, Royal Purple is not a good synthetic.
It's not a BAD oil, but it's nothing special.
http://personal.linkline.com/rlockyer/oil/oil.xls

In my motorcycle, Castrol GTX held it's viscosity better than Royal Purple, which had sheared down from 50wt to 30wt in only 2000 miles.

Personally, I use only Mobil-1 or Amsoil products for the engine and differentials. Toyota transmission fluid and coolant.