Reddit mentions of BG 44K Fuel System Cleaner Power Enhancer 11oz.

I'm guessing it's this stuff.

And no, it's not going to improve your gas mileage. It might help clean carbon deposits out of your fuel system and help your car run slightly more efficiently, but any MPG gains will be negligible. You're not going to go from 18 MPG to 25 MPG just by putting some of this in your tank.

-Catalytic converters will start throwing codes eventually. Had my sensors spaced out (put a spacer between the sensor and spot they screw in on in the catalytic converter) so it wouldn't throw the codes anymore. I think the OEM converters are roughly 900 each.

-Here is a discussion on the timing chain for the 4.2L V8 S5. It does not seem to be the same issue as the earlier 4.2L V8's in the S4 and RS4's.
http://www.audizine.com/forum/showthread.php/778497-Timing-Chain-service-at-100k-miles

-Carbon will be an issue, just no good rule of thumb on how long to wait before getting it delt with. If you start seeing miss fire codes probably time to get it checked. My local non-audi shop charged me 800 for the time to clean it with break cleaner and a brush. They charge 86 an hour. You'll also get charged for new gaskets to put the intake manifold back on. However I was getting miss fire codes after getting mine cleaned so I ran a bottle of this through my gas tank and never saw the issue again.
https://www.amazon.com/gp/product/B00DDVU6EI/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

-I've not had an issue with mounts yet not sure if/when I will. My car is currently at 111k miles and I bought it at 66k miles and I'm at least the 3rd owner. If you decide to replace them you can get more solid motor mounts, here is one company that sells them but there are several others out there that should sell them as well.
https://www.jhmotorsports.com/shop/catalog/index.php

-List of things I've had to get fixed that I can remember.

-headlights (If I recall it was about 250 for both new ones together)

-Carbon cleaning

-Stereo system went bad, had to go to dealership to fix it, 1400 for oem replacement, diagnosing and time to install ended up costing me 2k

-Replaced oil separator OEM roughly 350 hopefully cuts down on carbon build up. Considered an oil catch can just never pulled the trigger to do it. Gotta remember to empty those if you get one.

-Currently dealing with randomly leaking AC system. Shop put dye in the AC system when they refilled it twice now, still can't find where it's leaking out with the black light.

-Hood strut went bad, replaced it.

Basically all general maintenance stuff outside the audio system. If you have any other questions let me know.

If you regularly use brand name fuel like Chevron or Exxon, you can skip the injector cleaning well past 30k miles. If you regularly use the absolute cheapest gas you can find, you may way to use BG 44K on occasion. Probably works 90% as well as the dealer service at 15% the cost. https://www.amazon.com/BG-44K-Fuel-System-Cleaner/dp/B00DDVU6EI

What kind of gas are you running? What brand?

In turbo engines running cheap low grade gas can cause carbon buildup. You should be buying gas from top tier stations.

Run this through your truck a couple of times, should help.

BG 44K Fuel System Cleaner https://www.amazon.com/dp/B00DDVU6EI/ref=cm_sw_r_cp_apa_z4EKybBVCRTRY

I use two in my 00 toyota sienna.

http://www.amazon.com/BG-44K-Fuel-System-Cleaner/dp/B00DDVU6EI

http://www.amazon.com/MOA-Part-110-Engine-Supplement/dp/B00FX6N7DI/ref=pd_sim_auto_1?ie=UTF8&refRID=0DG7P6AQQ4C24AMTHVK7

Early on I had a gelled up engine, low oil pressure issue I used the BG cleaner and removed the oil pan to finish the cleaning. Its been pretty good!