I have a 2008 GTS that is my daily driver. Starts and runs great however recently I have noticed I can hear lifter knocking. Then check the oil and it is low. I have filled the oil to the correct level and after a long drive. Let's say 100 miles it has burned some oil. No leaks externally but honestly it seems to change it's own oil. By the time 3000 miles has come up for an oil change I have already added 5 quarts. Is this common?

 

Could be different things.
Does your car smoke (blue) when starting up cold?
With my car I had bad rings and a burned valve stem.
Check you pcv valve also
Previous owner didn't take care of the car, so I did a rebuild with a mechanic friend of mine.

 

Could be a couple things. If you're absolutely sure there are no external leaks then put a catch can on and see if that fills up. If it does, then your rings are shot, which I found to be the weakest link of the engine anyway. If that's not where it's coming from then it's either your head gasket or valve seals.