My 2017 F350,6.7,CC,DRW,4X4 has a 14,00lb. GVWR. Also the 250 & 350 don't have the same suspension if DRW F350. In California registration you have to worry about "GCWR" which is "Gross Carrying Weight Rating"
I might get corrected but, 28,600 is the GCWR Gross Combination Weight Rating & 20,600 is the towing rate max.
Say the vehicle weight 8,000 + max towing 20,600 = 28,600.
I'm using what I did for my 2017 F350 CC 6.7L 4X4, DWR & GCWR.
I've got 80,000+ of the 102,300+ miles towing a 37ft. 1/5whl at max. gross weight, 21,000lbs. I had Ford service the trans every 15 - 18,000 miles & not iota problem.
I use a plastic 8 oz. baby bottle to measure the correct amount of additive. 32 oz./100 gal = 0.32 oz/1 gal or 3.2 oz/10 gal, & 6.4 oz/20 gal, etc.:sweet