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I regularly towed about 11000# with my 8V92 making gross weight about 55000#. Drivetrain was ok but you had to be careful about coolant temp (gear down, slow down, and keep revs up) and diff temp. Run full synthetic in all. I had no fan clutch on cooling fan and had installed the 9 blade series 60 fan.
I regularly tow 20000# with my series 60 grossing at about 70000#. Cooling is not an issue on the series 60 as the computer handles it but diff temp still is even though it is a larger axle. Slowing to 60-65 handles diff temp. Make sure you install a gauge if you don’t have one.
I tried to post a photo of rig. Note the front axle on the trailer…virtually no tongue weight. Competitive built about a dozen of these.
All the above said, I push the envelope all the time but I agree with most that you’d probably be better off with a series 60 coach and the larger diff. Not sure when it was phased in but my 1989 took 3 gallons on a diff oil change and my 1999 chassis takes 5 gallons.