you have seen it? i dont think thats possible...when that much oil burns, there is smoke. end of story. i would believe a teaspoon a day not causing any smoke...but 3-4 quarts per week, from a 6cyl? (especially one as small as a 2.5L) no...if it is burning that much oil inside the engine, there has to be smoke...simple as that. now if it is leaking onto the block somewhere and then burning off, then it will smell funny but no smoke.
heres what you should do, drive the car around the block. dont drive like an ass, just do normal everyday driving. when you get home, get out and open the hood. look for fresh oil. also sniff around for an oil smell. then LET IT COOL FIRST! and get down by the tailpipe and run you index finger along the inside of the tailpipe. if it comes out oily, or with excessive soot, you
are burning the oil in the chamber and attention should be paid.
but again, i do not think its pssible to burn that much oil, and not get smoke or at least a smell. and by now, his cat would be toast, so it should be smoking. it is my opinion, that you have blown a head gasket. but even that should emit a blue/black smoke. coolant burns white. oil burns blue/black, so when you blow a head gasket, oil and coolant mix together in the cobustion chamber and burn. and presto...blue smoke is created.
not to be a jerk or anything, i just dont think its possible.
pull your spark plugs and examine the electrodes to see if you are burning oil 
"Modified? this little 6 cyl? Dont be upset because I beat you!"
