I have used Mobil 1 10W-30, Synpower 10W-30, Delvac 1, Syntec 5W-40, and Delo 400 15W-40 in my 94 Toyota pickup(22RE). I calculated the average mpg over their interval's and the data is below: C. Syntec 5W-40... 27.2 mpg Valvoline 10W-30... 26.2 Delvac 1 5W-40... 26.0 Mobil 1 10W-30... 25.5 Delo 15W-40... 25.3 V. Synpower 10W-30... 24.1 All intervals were a combination of warm and cold weather except the Syntec which was run from April through November(I believe this explains the jump). No mechanical problems or major tune ups during these intervals. Also, the Delo sample saw very little highway driving which had to hurt its average some. All samples were run between 3 and 4K miles. Anyway, the point of my post is that my records on this vehicle do not support the claim that lower viscosity oils and/or synthetic oils provide better fuel economy(of any significance). I was surprised. FYI I'm currently about 2000 miles into an interval of Mystik 10W-30, and my average is a lousy 22.6 mpg, with more than normal highway driving. I will be testing Pennzoil HD30 this summer.