SD if your truck idles and runs smoothly that is about all one can hope for.. Changing to new plugs or wires won't help at all!
But I will attempt an answer.
Use the sparks that are the exact factory installs.. Using a different brand cannot ever be better and in many cases will be worse than the factory plugs.
Then for filters..whatever is available in stock form at the parts house..
OIL is your choice..NO BRAND will cause better gas milage. Oil simply lubricates the bearings. Look at all the fancy oil ads and then make your choice.. I use whatever is on sale as I believe all oils are the same.
There is no reason to change the plug gap from the factory specs.
Now to the meat of the matter. My 02 4 litre has 5 gears. I almost always use only 3 of them!! I only use all 5 when entering an expressway from a stop. In normal traffic I use only 1, 3, and 5.. Or sometimes If the street is slightly downhill I use 2, 4, and 5..
I keep the rpm low never going to 3,000! I shift at 1600 to 2000 rpm. My milage in the Texas heat with the AC running full blast is 18 on every tank! I havn't had it on the hiway this summer but I would expect a solid 20 on a trip.
So the advice here is to keep your FOOT out of it and see what happens to the milage.
Big JIm