I have a GTA now!
Join Date: Oct 2011
My two cents worth.. The JTS uses a fair amount of oil for a number of reasons.. Some of them are bad oil rings and higher compression.
Also, if most of your trips are under, say.. 20 minutes you'll get a lot of oil contamination - Fuel, gasses, moisture etc. That only burns off on a long drive of over 30 minutes, the longer the better.
So you may not notice the oil level move much at all over an extended period, but on that first long drive you'll lose a litre in a few hundred km. Thats all the crap in the oil burning off.
David's right, check your oil more often and make sure you have a high quality fully synthetic 10w60 in the engine.