Whilst fuel doesn't sit about long these days it does degrade in quality with time and the octane level drops off as it sits in storage.
I think if your car is standard and you have NO intention of modding it then 95RON is fine. You only really benefit from the 98 at high RPM when you drive the car hard so unless you thrash the **** off it every time you use it then it's of limited benefit.
Some of the expensive fuels claim to have better detergent properties, but given many of the engine internal photos I've seen there is no clear evidence that this is doing much for the valves, so again I'm marginal about these claims. I do wonder whether there are any negatives to fuel like Momentum and whether over time they could cause seals in the fuel system to perish, but again there is no evidence of this on the net. Don't expect night and day difference, it is very subtle. Some will claim smoother running from Shell fuel, but if all things are equal your car should run OK if all other things engine wise are in good nick.
HOWEVER, if you have your car remapped (esp. for a remap where you can adjust fuelling and timing - e.g. with a Revo SPS), then your car will be mapped to benefit from the extra octane. I was buying 100RON in Germany and you could feel a difference in the cars top end urgency, whereas that difference may have been marginalised in a non mapped car.
One thing I wouldn't do is switch petrols between high and normal octane as otherwise the ECU will be constantly having to adapt to new parameters, so choose one and try and stick to it for consistent motoring. Personally I use Shell V-Power wherever possible and I've never had a bad batch as far as I can tell.
and was told by the previous owner he was advised by Audi to use the pricey stuff... In my area i've got the choice of Momentum at Tesco or normal fuel.
