I second the Lucas fuel additive, my father uses it on his 1997 Peterbuilt with Cat Engine and I use it on my 2002 ST. Improves engine response, supposed to help fuel burn more efficient cleans out the intakes and such and has given me constant on the highway mileage of between 20-22 on a speed average of 75 mph.
I also use Lucas Motor Oil additive in place of 1qt of oil every oil change mixed in with Mobil 1 Syntec or High mileage clean 5000 and Lucas Transmission and gear additive My ST has 116148 miles on her and no engine, fuel, gear or transmission problems.
My father also has used such additives on his PETE/CAT and has had no major problems, went well over 1 million miles before a "complete engine refub" done as a "tune up", not because it needed to be repaired/fix.
Edit: I must note that I do have a free flow custom exhaust & K&N filter which may play a part in the constant 20-22 highway miles average, but I do notice a differance of about 1.5 or so on trips I don't use the Lucas Fuel additive and the truck seems not to pur as smoothly when just running on the 87 oct.