I wish I knew exactly what happened to my oil consumption, but here is my story.
Bought a used 99 ML320 from a friend. 90,000 miles. He said it used a quart every 1500 miles. And it did for me. But then I cleaned my plugged EGR tube and ran 2 bottles of gummout emmission contral cleaner thru it, changed the oil and the consumption went to 1 quart every 4,500 miles.

1st owner used mobil one as I do.

1/29/04 UPDATE
4500 miles since my last oil change and the oil is right in the middle of the dip stick. So now it is using less oil as it gets older. One significant change I did from the origional owner is, he burned regular gas. I only burn Mobil or Shell premium gas. So perhaps when the computer retards the timing due to cheap gas, this throws the engine off enough to suck oil into the intake.

1/30/05 update- Thank you Nikki for the contribution a couple down from this.
It made me think, the letter talks about ring sticking due to sludge. That makes more since now than anything else why my ML320 used oil then didint. I had a ring that unstuck. I am going to continue with gas additives weather they work or not.

__________________
Steve Danville
what i did myself.
CPS
rear shocks
MAS x 3.
SparkPlugs (twice)
Fuel pump & Filter
Window switch
replaced both failed horns with Honda horns.
Cleaned ERG Tube
Oil changes
Oil consumption Fix
Brake Pads
A/C Recharge
Anti-Freeze change X 2.
Front Shocks. Monroe. $90
Mercedes Warrenty Work:
Harmonic balancer,power steering clamp. Cat converters.Drive Shaft Bearing.s
Replaced Outer Lower Tie Rods and front lower ball joints at 156,000 miles