I recently purchased a 1996 GMC Yukon (5.7L Vortec) with 118000 miles on the clock. It has probably been run with 5W30 from day one, but my mechanic who did a transmission flush today has recommended higher blends for the miles on it: 10W30, 15W30 maybe even 10W40. Is this sound advise? His suggestion is Quaker State Higher Mileage because it has all of the additives in it already, and says that for a truck of this mileage he does not recommend full synthetic as it runs too thin. The truck does have a slight lifter tap, but nothing outrageous. I'm not so concerned about the brand as I am about the blend and the dino/synthetic comments. What's the take of the masses on his thoughs?