I have a 2006 coupe A6 with the LS2 and have 174000 km on the clock.
The only issue that caused any grief was the water pump seal leaking at 50000km.
Regular maintenance items replaced over time and mileage include:
Shocks at 120000 with Bilstein B8
Brake pads at 60000 to ceramics, and just got a safety inspection, and still 3/4 pad thickness.
Sway bar end links
Changed out the fluids this Spring. Used OEM fluids. Tried Redline with additive for the rear diff-too thick as the clutch packs would not lubricate. Changed to Dexron, and noise immediately went away.
Changed the rear axle nuts, only because I was doing the diff fluid change as well.
4 batteries 1 OEM, 1 Optima yellow top that only lasted 6 months, and the last 2 from CTC
Got rid of the Goodyear tires before year 1 was finished. Terrible tire, and dangerous in the wet.
Changed to non runflats- 3 sets of Firestone Indy 500 Wide Oval
Latest set is Michelin PS4S
Changed out all TPMS sensors at 10 year with a new set of tires. It was time, but they were still working OK.
Change oil dependent on mileage or minimum Once per season.
During some summers and long road trips, changed twice.
Tied back the shifter lock out pawl
No issues with the Harmonic Balancer
No issues with the gas tank top fitting crack and fumes escape (mostly happens to 2008 and newer)
No issue with the low beam headlight - did not get the recall done. I will fix myself if need be.
No issues with the driver seat air bag connector fretting causing air bag alarm -zip tied the connector so did
not need the safety recall to get done. BTW, GM removes the connector, and solders the wires together as the fix.
If you ever need to remove the seat, you will have to cut the wires, then resolder.