Guys, i have three GM trucks that have 4.3, 4.3 , 5.7. The 93 C1500WT 4.3 has 212k miles on it now and spent the first 97k miles in PA as a company work truck. It was cared for pretty well but ran nothing but fleet 15w40, yes even in the 0F winters. Once i bought it i used it to drive to college and work. Since i bought it i have put about 115k miles on it. Mostly using rotella 15w40. Truck still runs great. im starting to notice a little knocking upon start up lately. Maybe due to its long use of 15w40 in cold pa weather, im not sure. it burns about a qt/4k miles. I have tried some 5w30 conventionals and really have not noticed a fuel mileage increase. I track mileage every single fill up and have for the last 5 yrs so i would notice. I was also driving about 100 miles a day on the interstate so oil was warm for most of the miles. My blazer with a 4.3 is a different story, I bought it new in 01 and have run Mobil 1 5w30 since day one. At 175k miles it had a LIMG leak and stuck up a lifter and sludged up the engine a little. After fixing that i ran some 15w40 thru it to clean it up a little and it being spring in NC at the time I felt comfortable doing so. We use this truck as a run around truck ,it gets many short trips, and i definately noticed an increase in mpg when i drained the w40 out and refilled with syntec 10w30. Nothing special to say about the 350, so i wont say anything. My point is that you can probably use any darn thing you want if the conditions are correct. If you are doing alot of short trips in summer, you will notice a decrease in mpg, maybe slight, but it will be there. If it is winter, it will be very noticable. My blazer as an avg changed 1.6 mpg with the thinner oil. Like i said it was many short trips. If you were to driving many long trips, i dont think you would notice the difference in the 15w40 vs xw30. This is even more true for the lower reving v8s vs the v6's. Im not recomending anything but just making a point that depending on conditions, anything from 5w20 to 15w40 will work very well in these engines. One thing to keep in mind about high mileage oils is they "generally" have a little higher HT/HS than the regular. I have done a lot of experiments and research on oils and would like to say to people that are using 15w40 in <20F temps, throw a bottle of rotella and a bottle of 5w30 conventional and a bottle of 5w30syn in the freezer, then make your decision. Side note is i just read a post on here from a guy that used 20w50 in his tahoe for a long time and things are still going well. It would be interesting to know how much extra money he spent in fuel over the last 5 years though.