2003 Silverado 1500HD 6.0 gas 11,000 miles Last winter I started to "hoard" mass quantities of green GC. My intention was to us GC in my truck year round for many years. It was suppose to be the ultimate oil for my climate and my vortec 6.0 Well about a month ago my supply of M1 SL SS finally dried up so in went the GC along with a upf-44 delco filter and a maintenance dose of auto-rx I initially noticed a much quieter engine. I was a happy camper. Then, about 50 miles into my GC change I started hearing what sounded like a lifter(s) rattling. The noise was most noticeable at the first shift point. You would hear it immediately before the truck would shift, say around 1500 rpm, and for about 1-2 seconds as the RPMs dropped. The noise was upper engine related and not CSK ( I know what that sounds like). It would not do it until the truck was about 1/4 of the way warmed up, then it would do it from then on. You could also duplicate it by reaving the engine while parked. Putting the truck in tow mode seamed to quiet it down. I ran the GC for about 200 miles, cause I have heard some say it takes the engine a while to get used to GC. Finally I got ****** of enough so hear is what I did: 1. Thinking that it could be the GC or auto-rx I dumped the fresh GC !! but left the filter alone and put in some cheap .49/quart chevron 5w30. This did absoultely nothing. I ran this for about another 50 miles. 2. Changed the upf-44 declo filter, leaving the cehvron in the pan, with a hastings. NO MORE SOUND!! I have been noise free for the past 200 miles. So, now I feel really guilty for blaming the GC and dumping 5.5 quarts in a moment of frustration. I had also tossed the old filter. Should have kept it. But I am happy once again, and I still have 80 quarts of GC in safe underground climate controlled storage. Any thoughts?