Tips for getting an entire A4 TDI to 500k?
By far the most important maintenance item for your TDI is premium quality diesel fuel. Find a station that sells Amoco Premier or BP Supreme, or some other verified good quality fuel. Use fuel additive EVERY tank. Also try running 20% biodiesel, weather permitting. Quality fuel protects the fuel pump, prevents soot buildup in the oil and intake, and keeps your injectors firing properly. It's so much more important than oil, it isn't even funny.
You can run good quality oil to 10-15k miles no problem (given good fuel). Get a bypass filter and go even farther. Make sure the car heats up to full operating temp every time you drive it (meaning no short trips - under 15 miles). Most of the wear on any engine (gas or diesel) comes from cold starting, so start it up as infrequently as possible.
Your shocks will wear out in 50k miles. Wheel bearings will go eventually, and so will alot of other stuff. Just replace em as they wear out and the car will drive well for as long as you want it to.
As for "what fuel", I consistently get the best mpg from the BP diesel or the Phillips 66 diesel. My 2 best tanks, 54 & 56 mpg were both with Phillips 66 diesel. My Jetta typically averages 52 mpg with either of the above on the interstate at a 71mph speed - in warm weather. I consistently get 2-3 mpg less with the Marathon diesel. The Pilot diesel seems to be comparable or maybe just a bit better than the Marathon. Haven't tried the Sunoco diesel yet. I live in IN, but, have found the BP diesel in IN, IL, OH, and MO. I've found the Phillips 66 in the same states plus WY, KS, and CO. Out west, you'll also see Conoco gas stations, and, further west, you'll run into Union 76. All 3 of these chains are owned by Conoco-Phillips. I am not sure whether the same diesel fuel is common to all three chains. COP has several different refineries, so, I just don't know. I also recall reading that Arco (pacific coast) is owned by BP. Again, don't know if both chains use the same quality diesel. I can tell you, though, I take several long trips out west every summer and I see consistent mpg's - as long as I run the BP or Phillips 66 stuff.
As for additives - there seems to be a vast difference of opinion on this site as to whether we should run them or not. I would certainly like to know ! Go to the fuels forums and 1 person says you should - clean injectors - better mileage - etc. The very next post says no you shouldn't - dirty injectors - reduced injection pump life - and you get the picture. I've spent much time reading those threads and cannot figure out whether I should run an additive or not. In my job, I do alot of data collecting and sorting, so, I'm only interested in opinions with meaningful data to back it up. And would very much like to see some. I keep entertaining the idea of running an additive, but, just don't know whether or not its beneficial or safe. Even in this thread someone says it prolongs the life of the injector pump. In another thread I read recently here, a person was saying that most injector pump failures were a result of running additves.....??????????????????????????????????
The TDI is perfect for my several fishing trips and that's what I specifically bought it for. I too, want to drive it for 20+ years and 500+k. I cleaned my intake last month and am installing the ProVent filter in the next few days.......