Last oil change on my 3.4 V-6 GM engine, I put in Havoline 5w40 Synthetic (Big Lot Special). 5w30 is recommended, but for the summer, I thought I'd give this a try. Well, while mileage didn't suffer at all (did just as well), the engine seemed bogged down or less peppy. Changed the oil yesterday and put in 5w30 Valvoline Synthetic and the engine seemed to have a little more life to it. It wasn't just me, the wife had complained about this too. I'll see if she notices any difference now that I've changed oil. Any thoughts? Thanks.