For what it's worth, I just spoke with Curtis at Permatex and grilled him for info on different sealant options. He clarified that the Ultra Copper is designed for exhaust applications with extreme heat. Ultra Black is designed for applications where gaskets don't quite cover gaps due to poor machining on older Ford and Chevy engines. He confirmed that Ultra Grey is designed for applications like the tip of the front cover seal - high tensile strength, high vibration and flex tolerance. The MB clear RTV (002 989 45 20 10) seems to be harder and harder to come by and getting more expensive when it can be found. Hope this helps.