It does depent on the ambient temperature your car will generally operate at, as Im in Northern Australia, and with the hot temps, I would put heavier oils like 10-50 or 15-50 into my A6. I am assuming you dont have your manual, but if you do there is a chart in the D-I-Y section.
I was told that it also depends on whether you are on a fixed or variable service interval. If you are on a varibale service, then you need to use a very thin oil as the ECU takes readings from a laser whcih is passed through the oil sump to monitor the condition of the oil and help determine when the next service is due.
Mines a 2001 2.4v6 and I was recommended Castrol SLX LongLife (which has now changed to a slightly different name over here in the UK) - this is a 0W 30 grade oil.
Mine also drinks oil quite a bit (every 2000 miles or so, the light comes on and I put another litre in) but I'm told by the dealer that this is normal for this grade of oil.
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