I am looking for a new tow truck.I have never owned a diesel truck.Will the new trucks run on the old diesel fuel as well as the new?Will the old trucks need an aditive?Will the new motors get better gas milage?Will the new or old fuel be hard to find?

The new diesel you are talking about is Ultra Low Sulfur Diesel or ULSD. The new (07-newer) diesel engines will only run on this diesel, unless the dealer tells you otherwise. I have heard that the first of the 07's will work with the old diesel but you will have to take them in later in 07 to have the conversion done. The older (06-older) diesel engines can run on both the old diesel and the ULSD. I have been running some of the ULSD in my cummins and haven't really noticed a power decrease, but I have noticed a little loss in mileage. When I first heard about the change in diesel everyone was saying that it wouldn't be in stations till early 07, however I'm seeing more and more stations with it already.

Hope this helps.

Early '07s will be low sulphur, but mid (like Febuary) '07 trucks will be ULSD. No conversion will need to be done, since the older trucks will run on the ULSD. The newer trucks however wont run on the old low sulphur diesel.
Mileage is said to increase with the new trucks, however the new ULSD contains less BTUs than the older, so it wont be a factor in increased mileage or power, like Blown06 said, older trucks will probably lose mileage and power. The newer truck's extra power and mileage will be from the addition of EGR and other refinements to the fuel system. As with all new diesel trucks, you will probably need to put 30,000 miles on it to see the advertised mileage that everyone claims.
Amsoil is one company that makes the new 5W-40 which is API CJ-4 certified to work with the newer diesel engines.. They are also in the works of making an additive to restore lost power and mileage from running ULSD in older trucks..
The new fuel is hard to come by in some places, mainly towns outside of metropolitan areas.