I used to run Mobil 1. Not the euro stuff as I could never find it, I just ran the 10-30 and changed it every 5k. It worked fine and all, but I didn't like to 10 factor or the 30 factor very much so I looked for something different. I was not about to rely on special order oils, so I never explored the Motul, Lubri/Liquimoly, Amsoil (although Amsoil is getting ready to go in my B6's 02x), or any of those. I don't want to be on a trip somewhere in need of an oil change, or even just a quart of oil, and not have one handy. I've always been a big fan of HD Diesel oils due to their resistance to shear, ability to withstand high temps, and high detergent content all of which being very good things to have if its going into a 1.8T. I spent a lot of time over on BITOG and ended up with 3 candidates. Shell Rotella T6, German Castrol, and Mobil Delvac 1 ESP.
German Castrol (supposedly available at auto parts stores) I have yet to actually come across... though I have not looked too hard for it.
Mobil Delvac 1 ESP (available at most truck stops) is a true Group IV HD Diesel oil with fantastic specs. As much as I hate to admit it, from the specs I have seen, it appears to be ever so slightly better at everything than Rotella T6. (2-3 deg here and there but overall minuscule.) Mobil 1 "TDT" (available at Walmart) from what I read is virtually the same exact formulation.
Rotella T6 is a cracked hydrocarbon and I guess is technically a Group III (last I knew) but it kills traditional Group IIIs (and most Group IVs) in every way. Don't expect it to break down and sludge up your engine any time soon. It's actually pretty effective at cleaning it. It's marketed as a synthetic, carries the "SM" rating and all that jazz. I've seen nothing but rock solid oil analysis reports from it. I know this is not a 1.8t and I don't have one for that motor as I don't put enough miles on it to warrant one, but here's a report from my 2.7T with over 7k miles, lots of stop and go, several 2hr rtips of sustained 3krpms+, a full summer, and part of a winter on it... and no make up oil to dilute any results.
As stated, I'm a fan. I ran it in my B5 (1.8) when we had it, it's in my B6 (1.8), as well as our C5 (2.7). I run Rotella T6 in everything, hell I even eat a teaspoon of it a day to stay regular and ease joint pain.
Seriously though... upon switching to it, both my 1.8t and my 2.7t got quieter (for certain) and seemed to run smoother (possibly a placebo)
Not going to bag on any of the foreign or boutique oils, but don't overlook whats on your local shelves... some of it's very good stuff.