Exhaust fumes on 2001 Cadillac DeVille DTS

Since driving out of the showroom in 2001, the exhaust of my DTS has been stinking like an old diesel army truck!
GM service never could do anything about it. Oil level goes down by 30% every oil change. Is that related?
Is the smell from my exhaust that of burning oil?

no. normal oil consumption, according to gm, is a quart every 900 miles. so, adding 3 quarts between changes is normal.
the cats are known to stink of rotten eggs. usually, there is always a cause to the converter doing this, such as rich fuel mixtures. surprizing that after all these years no one could solve it for you.

Thanks Roy. So could be a matter of software upgrade? Or sensor inside cat converter malfuncttioning? Fuel consumption is normal. Is that not inconsistent with rich fuel mixture? Marwan