Hey guys, I just found an interesting thing about GC Green. It fluoresced very bright green when hit with a uv light. I have a little uv pin light sold at AZ and other auto parts stores that comes in a AC leak dye kit. I checked M1 and it shows a dull brown color. I don't have any GC Gold so I can't say about it. Anyone out there have some GC Gold and a uv light? This might be another test for verifying the good stuff. At the very least, you can track down that oil leak that has been bugging you. BTW: GC Green doesn't fluoresce through the sight line on the bottles. That rules out using it in the stores as a secondary check to the batch numbers.