Keep in mind that the best way to "mod" your truck is to take all of the responses and go with what you come to conclude is your best option. You are on the right track with forums of actual owners. Watch out for who I call "vultures" that sell product and troll forums offering no real advice other than selling what they have. These are my opinions:
2. Synthetics in our 7.3's has no benefit, kind of like using 93 octane gas in a car requiring 87, it just costs more. Synthetics should not prolong your oil change interval. Oil and oil filter change every 5,000 miles with a fuel filter change every 3rd oil change or 15,000 miles. Use ALL Motorcraft filters in your truck. Air FA 1750 ($20), Oil FL 1995 ($10), Fuel FD 4596 (you DO catch a break here. go to Racor PFF4596 Fuel Filter / Water Separator
You can buy the Racor 4596 for $16.59. (last time I bought from there, over $50 was free shipping so I got 3 filters, Racor makes the fuel filters for Ford so this is the EXACT SAME filter without the Motorcraft box at half the price)
3. The only fluid that has to be synthetic is the rear differential. All other fluids should be changed and start a scheduled maintenance chart. My trick for oil changes is that I always change my oil when the thousands position on the odometer reaches either a 5 or 0. ex. 145
,000 and next at 150
,000 Also, consider switching to red ELC coolant when you flush it. SCA levels MUST be controlled in these trucks by using additives and test strips. Using red HD ELC coolant not only lasts 300,000 miles before maintenance, it requires NO SCA attention. Well worth the switch.
4. If you have a floor shifter for 4X4, you have manual hubs. If they won't move, they are probably frozen. Take them off and clean them or it may be better to just replace them. Ford arrows wear off very quickly
5. The best thing for MPG is really to drive like you have an egg under the accelerator. Get rid of the muffler for a new exhaust system or a "flow through" muffler. Some people swear by performance upgrades, I like my truck with very subtle and minor ones. It depends on you. Just consider, will the results be worth the costs and/or risks?
6. Intake and exhaust should go together. DO NOT get a tuner without gauges and know the maximums and minimums of the gauges relating to the truck. The more powerful you make the truck, the stronger other parts must be or there will be a failure somewhere. So, be careful about what you decide. Talk to someone who has nothing to gain that has "been there, done that".
To make your truck reliable and overall best mpg, my suggestion is to replace your muffler and use the Ford stock air box with the FA 1750 or buy the Ford AIS kit (around $200), follow a scheduled maintenance plan STRICTLY, never skimp on quality of parts (use OEM), pump tires to maximum psi while still wearing correctly, and drive it like you want to keep it forever.
Remember, we all were diesel beginners once and you are definitely headed in the right direction by taking the initiative to get this information. The 7.3 is the last of the great and simple diesel engines so keep it and take care of it! Good luck