Having been there, done that, I'm surprised this still gets the big ? that it does, as often as it comes up in the Troubleshooting Forum. I have wondered myself how mine ran on 1 quart without blowing, but it did, multiple times. Emptied itself on the way home from just having it changed, in 200 miles. Got to the house, pulled the stick, nothing. Took 4 quarts to get a full reading on the stick. Engine damage? Probably. It scares me how many times only a quart ran out when it was getting changed. However, I did my homework and solved the problem. Now I only use about 2 qts every 3500 miles. Here's how:

BIG #1. Clean the egr system every 25k miles. About $200. As carbon builds, the computer gradually richens the fuel mixture to compensate. This dilutes the oil with fuel when it washes past the rings. Hence, the unreadabe dipstick. Oil's just too thin to read. Notice that you can read it fine when you just did your oil change. After 100 miles of driving, it's unreadable again. Isuzu claims this to be the main reason why 3.5's blow their bottom ends, be it a Trooper, Rodeo, or Vehicross. They just get run out of oil.

#2. Don't have an Isuzu dealer near by, go to a GM dealer. My local GM dealer (Hudson in Silverthorne) services mine and the other 2 VehiCross's in my imediate area. They joke that they have become the designated VehiCross service center for this area. All 3 of us have been there at the same time. They did my EGR service, replaced my EGR valve, both upstream Oxygen sensors, and changed my oil. Every part, including the oil filter, came out of their stock. My MAF is even a GM part that came off their shelf. Every Isuzu part has a GM part# on it as well. Not all of them are viable GM #'s but most ignition, fuel injection, and emission parts are. They are always cheaper than their Isuzu counterpart, but still OEM.

Their tranny guy just rebuilt my T-case and installed it right before I went to Moab for Memorial weekend this year. After all, a Canyon/Colorado pickup is an Isuzu I280/I290 truck. The Trailblazer/Envoy is an Asender, etc. So it's very familiar territory to a GM tech.

So, get that EGR service asap and see how huge a difference it makes!