WARNING: This page is about a different car, the 2007 GMC Yukon XL and 2007 Chevrolet Suburban. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Using Straight Edge To Inspect Oil Pump Body Assembly Flatness
Courtesy of GENERAL MOTORS CORP.
Using a straight edge and a feeler gage, inspect the face of the oil pump body assembly (203) for flatness.