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Following application instructions matters. The Slip2000 EGL, for example, is supposed to bond to the steel. Such oils are usually polyester oils that bond electrostatically to the steel, but it can take a three-day soak for that to complete at room temperature.
The carbon test didn't impress me. I've seen much worse caked into M14 gas pistons and behind the piston head of a Garand op-rod. He didn't test any of my favorites for that purpose, nor metal fouling removal. If you're going to get into fouling removal tests, you really need to cover both area.