About a month ago, I was at the store doing my shopping, including getting some oil for my car. A man stopped me to talk about the oil that I had just purchased. As i turned out, he was a recently retired engineer from GM who worked with their various engines. He mentioned that the Valvoline 5w-30 turned out tops in the tests that they ran. He also said that none of the brands tested 10w-40 weight passed their tests. Unfortunately, as I was pressed for time, I didn't have the opportunity to ask him the 50 or so questions that I would have liked to have asked. I doubt that one use of the 10w-40 is going to cause any long term problems, but based on his advice, I would stay away from using it regularly.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.