I would suggest that you do a quality tune up with top shelf parts and then change the motor over to a high quality synthetic. Amsoil series 2000 0W30 will net you at least a couple of miles per gallon. With the bypass filter system you could run up to 20k miles between oil changes BUT you must still change the filters at mfg. recomm. intervals. 2-3K miles for the full flow filter, 7.5K for the bypass filter. Obviously you will need to top of the system with a quart or two when you change the filters.
Redline MTL in the manual trans (about 3 qt) and just over 2Qt of high quality gear lube in the rear diff. The K&N does allow more air in but it allows more dirt too. I would stick with the Toyota paper filter because it will increase the life of your oil and oil filters. Your truck also has a somewhat adjustable front ride height so I would recommend you set it as low as poss. to decrease drag. May need to realign if you do this.
Keep it under 65 or so and you should get better than 20-23 mpg highway.