Ballcoach

Ok so my CEL comes on. Take it to Audi they reset it and tell me the code is due to poor quality gasoline. I have only put premium in it so that was confusing. But they gave me a free bottle of whatever injector cleaner and all is good for a month or so. Then last week CEL is back on so I go to parts store buy a bottle of injector cleaner light goes off on its own in two days. Stays off for three days and is now back on. Am I going to constantly be buying cleaner to put in this thing. Also is it normal for this engine to use oil?