audi a5 engine oil

Good oil as also C2 .

Best value for money is Quantum LongLife III from ebay around £19 .

Realistically any oil with 507.00 on it is suitable .
 
Who was it that changed the grade of the oil? An Audi garage/specialist/local?

You could just ring Audi and ask if there has been a change in the recommended oil, if not, go by your car manual...