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????)