What fuel should I be using? 91 or 95 octane?

OK, I know the topic of fuel has been discussed a few times, I've searched on the forums but didn't get the answer I was looking for.

I will shortly be going to pick up my used 2001 S6 4.2. This was a special import car and the manual is only in German.

I have two choices of fuel here 91 and 95 octane. I don't know how the fuel octane is calculated though. US system or European system.

To be cautious I will use the 95 octane.

Anyway, the question, the question again.... what fuel should I be using? 91 or 95 and is that US or European?

Thanks for you help!!!

 

91 for the USA and 95 for Europe but in the 4.2 the higher the RON the more power it gives so 95 or better ( not sure if you can get that in the US but in the UK it starts at 95