I found a good deal on an 06 Escalade EXT, has 163,xxx miles on it, but one owner and Carfax shows maintenance was completed at a couple of different GM dealerships. It drove nice, everything works, except a small radiator leak. I brought it home and while removing the engine covers, air filter housing, and rad, I could see that it would be best to replace the rad hoses, serp and accessory belts, remove and clean the throttle body, etc... and the engine area was filthy. While cleaning it, I saw that the valve covers may have had a small leak due to build up around it so I decided to replace the gaskets. Here are the pics. No sludge or build it. Just heavy varnish. I assume the GM dealers used conventional oil. I just wiped up the under side of the valve covers with a Berrymans soaked rag which did a pretty good job at cleaning most of the varnish, replaced with Felpro gasket kit and reinstalled the covers.