Burning Engine Oil?

So this is something I just noticed over the weekend, my car burned up about 3 quarts of oil since the last oil change. There haven't been any visible leaks anywhere so I haven't been checking my oil regularly like I should anyway, stupid move. Well my car started having a valve lash/lifter noise about a week or two ago. This had happened previously but my mechanic used additives in the oil, let that run for a week, then did another oil change to get the additives out. Hadn't really heard the noise since then. As soon as I put the 3 quarts of oil in it goes away, so now I'm thinking they weren't sludged up, but starved for oil. I didn't see a noticeable change in the level of oil from Friday to Sunday, but from what I can tell this must be an internal leak. So what has been suggested to me by my mechanic is to do my next oil change, probably next weekend, with full synthetic oil and while we're doing it to check to see if there are any noticeable leaks under the car when we do that. Then just keep an eye on it and check it at every fillup to make sure the level isn't dropping too much. I don't notice the car driving poorly or any kind of loss of power. Only occasionally do I get a rough idle that works itself out and sometimes a little jumpy at low RPM. But I have a bad exhaust leak and seized up bad O2 sensor, so I've just assumed those were from that. I'm hoping whatever the problem is, is a minor issue because engine failure would be really bad financially for me right now. Has anyone else run into this same issue with the 2004 GTS?