That truck/engine probably needs more items checked out, and inspected, and rebuilt, and adjusted correctly, than you may have considered. I like the engine, but things wear out, too.{so won't be as simple as adjusting the air/fuel ratio control valve}

A. Need to pressurize the Air to Air Aftercooler (ATAAC Core), The side weld may be cracked and leaking lots of boost/air volume

B. Timing Advance unit probably needs rebuilt and adjusted properly, then set pin-timing about 1 - 1 1/4 degrees advanced. Need to verify operation/adjustment of timing advance unit with a CAT Dynamic Timing Indicator tool, also.[with these numbers, then graph it out, then readjust towards the upper max line on CAT's timing graph THRU-OUT the rpm range)

NOTE: Timing is the 2nd most important item for fuel economy. Driver is #1.

C. AFRC needs to be pressurized to insure its diaphragm/ internal shaft o-ring/ and rear gasket holds boost pressure. Next, it needs to be adjusted correctly (either by dial indicator or by the "smoke on acceleration" after it has been "cocked")

D. Nozzles need to be tested; good chance they leak fuel at the upper body o-ring and/or bleed screw. (do a valve adjustment, while you are in the 'topend")

E. The plungers and barrels of the fuel injection pump are probably "clam-shelled" = uneven mating surfaces, where they have to hold in high pressure fuel. They probably need "lapped" and/or replaced or update the top bonnets and bushings to the newer style=uses different sealing area to mate to the "barrel". 

F. Probably have the fuel pump set 2 - 2 1/2 full turns out, on each screw, from factory settings [This is were an old customer of ours ran for years, and got good fuel economy  (6.2-6.4 mpg) Seattle to Las Vegas run. He keep everything up and adjusted very precisely]

Your throttle shaft oil leak probably needs the updated bushing install= reamed out the alum housing(special tooling), then squarely install new bushing and new lip seal.

Well, I have probably spent 6000-8000 dollars by now.(or more)

Timing Advance units are often overlooked, by Non-CAT Dealer mechanics. Years ago, this was the trend we saw.

On his year, 1985, it should be the "boost-cocked" style of AFRC, so if the diaphragm is torn/ripped, then it will have lots of black smoke, during acceleration; and will NOT limit hp at all(it NEVER gets "cocked" to limit fuel/rack travel)

Around 1991, the serial number prefix "3ZJ" engines, had "oil-cocked" AFRC, then with oil pressure , they limit fuel until boost pushes on the diaphragm(assuming a good diaphragm), but if the diaphragm is blown/torn/ripped, then it DOES limit hp severely(if adjusted correctly, and not super "rich" AFRC setting)

Now, the newer "oil-cocked" AFRC do fit older engines/governors, so anything is possible, depending on what is on his engine.

ATAAC cores, We use AV-TEC(after-market), if available for that truck model. We have had good luck, but one of my ex-coworkers in Omaha, said they(CAT dealer) went back to buying OEM ATAAC cores and had better luck with those for reliability.

The guy, I mentioned from Seattle to Las Vegas, tried lower fuel settings= more shifting and little less MPG. He immediately went back to higher fuel settings. NOTE: That was the only thing he changed, for that one trip.

Also, he drove it nice and smooth, but 65-70 mph. Just not a hotrod driver. (no shoes allowed in his cab, he only wore socks in his cab. Towels covered the whole floor)

Did I say very precise and picky, earlier?(about this driver/owner)

Your throttle shaft oil leak is just an oil leak, as long as your throttle linkage goes to "full throttle" and gets "breakover" at that pivot area, it will have no effect on power.

Another item, I just thought about, is that the flyweight races(inside the governor) probably are getting so worn, that you probably aren't receiving full HP at the very upper RPM range. Say 1900-2150 rpm, but you really shouldn't be running constantly in those rpms anyways.(all depends on when fuel governor was rebuilt last????)