So in terms of the warranty,
In terms of warranty, you must, must, must - must ONLY use oil which meets the standards listed in the owners handbook, of which 504.00 is the main one (but there are others).
I am ok to top up and replace the oil with longlife stuff. It costs more but is better?
The LongLife oils, which have the VW approvals are subjected to two further additional tests, over and above those tests required for non LongLife oils.
Normal, traditional multigrades, be they plain mineral oil, semi-synthetic, or fully synthetic - obviously go through a series of standard tests. The two relevent ones I'll try to explain;
firstly, to put it very simply, the oil is "poured" at two temperatures, namely 0 deg C, and 100 deg C - and measured to comply with long established criteria. These two temperatures are where the 20w50, or 5w30 numbers come from: the first part, the XXw is the performance at 0 deg C (and the "w", over the years has been loosly termed the "winter" performance), and the second numbers after the "w" are the performance at 100 deg C.
The second relevant criteria regards endurance testing - the oil is basically used continuously on an engine test bed for a pre-determined number of hours (fixed by ACEA test standards). The oil is then drained, and analyised. The oil must have maintained certain pre-test properties, one being the viscosity rating (XwYY).
Now, getting on to VW LongLife approval standards, regarding temperature testing, the oil is additionally tested at
150 dec C, and given a "High Temperature, High Sheer" (HTHS) rating. Furthermore, the endurance testing hours are actually double the number of hours over standard oils.
With that lot in mind, you can obviously (I hope

) understand why LongLife oils are more expensive. They obviously last twice as long as "normal"oils. However, the crucial bit to the GTI is the HTHS tests. Turbo chargers operate at very high temperatures, and normal mineral oils and even semi-synthetic oils "break down", developing very hard coke like substances. These deposits then gather in the oilways restricting flow, they can actually tear the filter medium in the oil filter thus allowing unfilterd oil to circulate, and finaly, the most damaging aspect is that they block the oil pick-up metal gauze strainer in the oil sump causing oil starvation. Now, most fully synthetic oils are much better at resisting this trait, but still not perfect. Most fully synthetic oils will break down at around 130 dec C, though some of the more expensive, higher grade fully synthetics do perform better still - Castrol Formula RS Performance and Protection 10w60 is just one example.
What I'm trying to say, is there is no guarantee that "any old fully synthetic" will be able to withstand the v.high temps in a turbo charger. It is for that reason alone, why I would strongly recommend sticking with the VW LongLife oils in a turbo engine, even if you use the time and distance servicing!
One final, crucial issue on longlife oils - there are currently three different manufacturer standards for longlife oils. These are VW, BMW and GM (Vauxhall/Opel). These standards are NOT interchangeable - do NOT use either a GM or BMW longlife oil in a VAG motor!
(no issues of mixing longlife synethetic with T+D semi?)
YES, there will be issues - don't do it. Semi-synthetic oils should NEVER be used in turbo charged petrol engines.