You can do a search on "VagCom" in the forum (plenty info) or use directly the Ross-tech
I have an USB cable purchased off EBay just as many others. You'll need a KII-USB cable and a 2x2 adapter
which you also can buy it off EBay.
As far as your oil pressure...hmmmm... Initially I wanted to say that the sensor may need be cleaned or replaced, but with everything else you have going on, and you mentioned, I refrained myself from doing so.
If the previous grade oil you used went past your rings like crazy - so you'd have no pressure/compression, then your cats would be messed up - no doubt about it. But the car would also smoke like crazy...which you didn't mention at all. Someone else could probably make a better sense of that for you than myself, I'm pretty sure.
(But you'd also have leaks all over, by all the seals..)
Running a thicker grade oil may not let it go past the rings; that could be one explanation.
Listen, I'm not a mechanic and my experience is so insignificant as compared to lots of guys around (I haven't "seen" some of them lately) that had their nose into cars and engines since forever. I could say that I blabber non-sense most of the time, even on subjects I may think I know something about.
However, having said that, I bet on all my Haynes non-shrinking neck t-shirts that you have something else going on beside the "oil" issue.
Scan the car first and start from there
Otherwise you'll just guess your way around.