2006 GMC Yukon Owner's Manual - Page 421

Also, if a vehicle has just been driven before checking the fluid level, it may appear lower than normal because fluid has traveled out along the axle tubes and has not drained back to the sump area. Therefore, a reading taken five minutes after the vehicle has been driven will appear to have a lower fluid level than a vehicle that has been stationary for an hour or two. Remember that the rear axle assembly must be supported to get a true reading. To get an accurate reading, the vehicle should be on a level surface. • The proper level for the 1500 Series is from 5/8 inch to 1 5/8 inch (15 mm to 40 mm) below the bottom of the filler plug hole. • The proper level for the 2500 Series is from 0 to 1/2 inch (0 to 13 mm) below the bottom of the filler plug hole. Add only enough fluid to reach the proper level. How to Check Lubricant What to Use To determine what kind of lubricant to use, see Recommended Fluids and Lubricants on page 6-13. 1500 Series shown, 2500 similar 5-47