I've used Bergey's in the past. Always had good luck there.
Good idea about the air filter.
How long have you had the truck? In other words, has this lower mileage just started? Are you trying to drive for mileage, or are you on the throttle a lot (80mph?). How many miles are you figuring into your calculation for MPG? I mean, are you figuring your mileage after only 50 or 100 miles? What kind of weight are you pulling?
1998 K2500 Suburban 6.5L TD 3.73 rear, Ron Schoolcraft 18:1, Kennedy ECM & IC, Timing gears, Splayed main caps, 3.5" Kennedy Exhuast/No Cat, K&N Filter, Boost/Tranny Temp/EGT(Pre Turbo), Ceramic-coated Manifolds, 195 Stat's, 265/75's (VSSB Adjusted) 7,000lbs (on a scale) Remote Mount Oil Filter, Remote Oil Pressure Sensor