Have been running the gold GC 0w30 in my impala with the 3.4 for the last month, Have driven about 1200 miles and noticed no decrease in gas milage, Have noticed though more engine noise at startup (lifters)I just had them replaced under warrenty about 5000 miles ago, and maybe my imagination but they seemed to be louder at startup after using GC, After warm up the motor runs nice and quiet and smooth, much better than when I was using mobile 1, I dont think that there is much different between 0w30 and 5w30 at least at startup but could that possibly be the problem ( to thin)? or maybe the lifters need to be readjusted after break in. Thanks