I wonder about the use of automotive products for the removal of carbon. Do the older products that were designed for gasoline 40 years ago (Leaded fuel) work better than the latest products are have been designed since we now use mostly unleaded fuel? Especially fuel with Ethanol, does it require as much carbon removal properties in a cleaner?
I'm just thinking that present day automotive cleaners might not be as effective on removing carbon since they are designed for newer gas, whereas the cleaners designed 50 years ago to remove carbon worked a lot better?
Maybe the best product might be gun cleaners that are designed for firearms in the present, not using an automotive product that was designed for todays gas? But I guess that was what OP was asking when he started this thread, asking "What is the BEST".
The Mercury product that was designed, I guess for outboard motors, would probably be more effective than a product designed for modern car engines with ethanol gas? Or as OP asked, is the GM product as effective?