You should only top up the oil with the same viscosity as what was original put in. Don't you have the service records or know where it was done? If it it's the long life service and was VAG serviced then I guess it will be long life fully synthetic 5-40 expensive stuff....and you can buy a litre from any VAG dealership. I have the same engine and I have taken my car off those crazy long service things. I do oil and filter change every 6k Miles with fully synthetic 5 - 40 Castrol Edge and now on Champion fully synth 5 - 40. I am lucky, my BGB engine so far ( touch wood ) isn't an oil burner.
Cheers
MM