Pretty much any additive will be sufficient from a reliable source. Couple of other ideas from expirence. Put a small plate of baking soda on driver and passenge side floor board to keep moisture under control. Pest bait all around the vehicle to keep from tearing up ducts, wires etc... If the truck is a standard get a stick or a rod to depress the clutch pedal about half way to keep the disk from getting stuck to the pressure plate and flywheel. This has happened to me on 67 Camaro, tried putting the transmission in nuetral and rocking the car till it came free but that did not work. I ended up pulling the transmission to un seize the disk off the flywheel. Now the pedal stays halfway depressed for a long stow. And the last thing I can think of is jacking the truck up (keeping suspension loaded to keep the springs from fatigue) to keep from flat spotting the tires (armorall the tires all over as well).
Hope this helps you,
Sean