When resupplying infantry, take the entire squad of ten and press examine on the truck. One guy from the squad will open up its inventory. When you take stuff out now, it will be dispersed among the whole squad, not just the guy going to the truck. The downside to this is that its hard to get small stacks of ammo, so you'll probably need 1 truck per squad. If you're resupplying tanks, you have to do them one at a time.