I have an '06 C9 with 982,000 miles never cracked open although it has always used oil. Just fueled up in JAX FL at 10.57 mpg, 4:30 gears, original Allison auto 6 spd, we run 58mph in 5th. Sub freezing temps, economy drops as low as mid 8's but never lower. Overall avg 9.5 long term. 65mph expect 8.9 avg. I service the trans annually with transynd fluid.
I have had good luck with the engine and trans compared to most. Before I learned to do overheads myself, I took it in for it's first, and Freightliner warrantied a turbo around 100,000, sd it was pumping oil but I did not see any evidence of that in charge air cooler, nor did they clean it out, me thinks they pumped the CAT warranty for some Christmas money instead. 1st HUEI pump failure I blame myself for getting in too big of a hurry and not letting the motor warm up to at least 140 as a pressure valve blew off and spewed oil out for a few miles. I am on my 3rd HUEI pump but the second only lasted 80,000mi.s or so the dealer said. CAT warrantied the 1st and the tow in, then warranty expired. CAT dealer may have missed diagnosed 2nd pump as the tech told me he had problems linking to the ECM. Also dunned me for a set of reman injectors for good measure. The HUIE pump is a precise instrument with pin bearings, electronic solenoids and valves thus the need to keep the oil clean. The ACERT technology in the C9 does not recirculate exhaust gases and does not have EGR, which I like, I would take a HUIE any day over any EGR set-up out there today.
Old school tech did tell me it likes clean straight 30W oil, but I run 5W15 Dello Synth, works better then Rotella Synth which seemed to clog the injector nozzles quicker. I like to pull injectors and gingerly clean the tips every 100-150,000 mi.s under a magnifier, replace with new o-rings. I attribute the injector tip clogging issue to oil consumption. I have learned to do the injector r&r myself with minimal tools. Had one injector cup leak coolant slightly and when I removed it, some trash was spotted in the o-ring seats on this particular cup. I assume this was cause for leak, probably that way from the factory. I did all new cups as a precaution, made my own tool using a large diam tap and steering wheel puller.
Other then regular maintenance, hoses, belts, thermostats, I have replaced a water pump twice and injector harness twice. Exhaust manifold started showing some leakage on side of head, torqued the nuts one ping, painted up the CAT yellow head and hasn't leaked since.
I ran into an owner of an '05 C9 with 1.4 million and no rebuild, but he said he was on his 8th??HUEI pump. He had no oil purification system. He sd he changes the pumps himself now, not too hard to do. He recommended I replace the regulator on top of air brake pump every so often, a small cheap enough part easy to do.
I will say CAT is proud of their parts, but haven't needed much in nearly 1 million miles relatively speaking, and I offset this by doing my own labor when I can. I find the motor easy to work on and plan to drop a CAT reman long block in it when this one fails. That said, it is running too smooth to think about a reman for now. I like the way this motor pulls the hills, much better power and fuel economy then a 3126 we drove for a year, but it had 5:11's if memory serves me, RPM's ran too high to me. We'll see what the future holds.