Help with purchase advice of 4.2 vs 4.0 v8

[3BOSH]

Audis here in Saudi are considered not very desirable when used (except for the new A8 for some reason and Q8)
I have been looking for a luxury car that has acceptable mileage and adaptive/active cruise control that works on its own in stop and go traffic.
it appears these all are applicable to A8s from 2012+ here according to the specs
in terms of priority to try to have some sort of an order I would say it is the comfort (suspension/seats/tire & wind sound deadening & decreased Noise/vibration/harshness) then reliability, then fuel economy.
I could not find 3.0TFSI car with the radar so it is either a 4.2fsi, 4.0TFSI, or s8 (4.0TFSI w/big turbos)
This is where I ask if the s8 is actually stiffer than the a8 and if so by how much.
It appears the 4.0 is better in mpg slightly than the 4.2, and almost identical to the S8. But from reading online it seems that MPG is misleading slightly as these cars age and have carbon build up so if you guy can share real conservative driving city MPG ( i got on the websites 13.6L/100KM 4.2, 13.1L A8 4.0, 14.4L S8) not in gallons to avoid UK/US confusion.
But now reliability comes into play, I have read online that on the 4.2 the timing chain is on the front on one side and on the back on the other so it is an engine out job.
PCV, coils, sparks I have read and watched everything I can find on the engine and the car itself so I am ok with consumables. I just don't want a major engine job or a catastrophic failure (unless I can inspect for it easily before purchase by myself or a garage)
like for most common jobs for the 4.2 and 4.0 like PCV's, oil coolers, oil\air\fuel separators, water pumps, thermostat, a/c compressor, and fuel pumps. Are any of these an engine out or as extensive of a job on either V8s? where is the timing chain on the 4.0 (hopefully at the front being just a pumper and some parts off job)
from my basic understanding it appears if you just buy a running 4.0 and reroute/delete the oil screen and change the PCV u basically have a breakdown free maintenance only engine?
w12 horrible mpg in the city, and has 4 more sparks and coils and worst of all if it is like the Bentley then it is almost engine out for everything.
side note: any advice on inspecting certain items is appreciated, I am not that familiar with turbos and Quattros & their maintenance as I am come from basic v6/v8 N/A cars but I got decent learning from past experiences and the internet obviously. We usually do not change the fluid if a car is past 150k KM if the transmission fluid was never changed.
Is higher mileages something to be worried of I test drove and inspected a 2012 300k km car that had recent maintenance it was honestly better and smoother than my 100k and 150k cars, it however had a slight rattle that could be a chain from cold start (left only for 1 day) but stayed for only 1 second. What I am asking is when should I need to change timing chains preventively and are high mileage rattles ok on cold? can't wait for the replies, no a8 diesel/electric/hybrid as far as I know ever came here.
"edit" I found 1 3.0 TFSI probably imported 2014 250k km 21 inch rims in case it is a better fit than the v8s
Thank you.
Hopefully becomes a new concise buyers guide

[UncleBen]

Man I wish someone had responded. I came here with a similar question and was excited to find your thread. I hope you got your answers somewhere.


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[MEnginerdS8]

Usually people are looking for the most fun, but you are clearly looking for some mechanical reliability as well.

3.0 would be most bulletproof reliability wise, But those have some little issues as well, not core to the long block , but they do take some maintenance on random crap like PCV systems, etc. 4.0T is pretty good overall too as far as newer euro vehicles go.

Timing chains on the older four point twos were definitely a problem. While essentially the same complicated 4 chain design, they have not been as much of a problem on the newer 4.0’s as part quality improved over the years through minor redesign of individual parts. I would not worry about timing chains on a 4.0 so long as you run the proper oil and do the proper oil changes at 5k mile intervals. Oil additive package and micro particles suspended in the oil lead to chain wear and elongation, but that can be controlled by being aware and prioritizing proper maintenance. You have to get into a newer EA825, 4.0 to fully solve the overly complicated timing chain conundrum.

I would expect better fuel mileage in a 4.0t w/ cyl deactivation than a 4.2 and also 3.0T, but your extra enjoyment of dipping into the 4.0T power on tap might negate that. A8L may just nudge the S8 on fuel economy due to the higher CR and same fun factor mentioned above.

[MEnginerdS8]

S8’s in US all had 21’s. You are much better off with 20 inch wheels or even 18 inch wheels that will make the biggest difference in ride quality. With same wheels the s8 in comfort mode should not be any stiffer than any of the A8’s, sport package or not. Theoretically, the longer wheel base would ride slightly nicer, but I don’t think you could notice that in the real world, that really comes down to the rear leg room, which is adequate in an a8/s8 and nice in an a8l.

[MEnginerdS8]

Injectors are about the only engine out job on a 4.0t, and most people discussing doing that, or talking about it for upgrade purposes for higher power, not because they feel injector went bad.

The W12’s are not actually all ‘engine out jobs’ on D4 chassis. Firewall has a removable section, bulkhead area if you will. Also there are many jobs people have figured out how to do short of book time. E.g. just loosen motor mounts and tilt vs engine out (still not ideal but…). They can actually be really reliable, you just won’t like the fuel consumption.