I am on the road just about every day.
I drive an F350 CC Dually w/ a 7.3 PSD & a manual ZF five speed tranny - usually I am towing 10,000 pounds.
I have tried oil additives over the years along with several brands and oil weight combinations.
Recently I ran across independent oil testing data performed & published by an individual:
Although lengthy & arduous it was fascinating to read.
The oil additive that consistently seemed to have proven measured results was Oil Extreme:
I called & talked with the owner Dan - then ordered a 16 ounce bottle.
For my truck I chose this oil from the test data link above:
113. 5W40 MOBIL 1 TURBO DIESEL TRUCK synthetic, API CJ-4, CI-4 Plus, CI-4, CH-4 and ACEA E7 = 74,312 psi
zinc = 1211 ppm
phos = 1168 ppm
moly = 2 ppm
I have been running Mobil 1 synthetic oil filters since they came out a couple years ago.
My engine takes 15 quarts so I added the recommended 15 ounces.
Then I drove a few hundred miles ....
I noticed an immediate gain in horsepower - fuel economy - performance.
I will be adding Oil Extreme to my ZF manual transmission & my 4:10 rear differential.