GM-GMC: Sudent Oil Consumtion, sanp on, compression check

Question
My 1998 Silverdao 1500 started to consume about 1 qt. ever 500 miles sudently about 6 month ago in a canada vacation. No noticeable changes on power etc, only a check engine light sometimes with sugestion to a "small leak in evaporator system"
What can be the source, or what to check 1st. I own a Sanp-On Acanner but with only 1998 cartridge for the throubleshooter, however I have 1998 cartridge for access engine functions and codes.
Thanks, Wilson

Answer
Obviously, the first thing to check for is leaks.

Rocker arm covers can leak and the oil will burn away on the exhaust manifold.  Rear main seals can leak only when the engine runs and leave very little evidence of leakage.

Next, check (or replace) the PCV valve.  Oil can be drawn through the PCV valve and then burned in the cylinders without any smoke from the exhaust.

Pull the spark plugs and look for any build-up on the electrodes.  Valve guide seals are usually the culprit, but the rings can cause loss of oil, too.  A compression check will verify the condition of the rings.