Do you have a true dual exhaust? If it is just a Y then saying the left muffler smoking is not going to help much since that is probably just the side your exhaust seems to flow better through.
A video of it smoking would help. "Light gray" is not a color I am familiar with when it comes to automobiles. Black is going to be a rich condition, white would be coolant, and blue for oil. MAYBE you are burning a combination of things. If it is constantly smoking you should see it in the plugs. Maybe take a temperature gun and check each runner, if you can see some sort of outlier with the temperature then you might be able to narrow the issue down to a particular cylinder?
A video of it smoking would help. "Light gray" is not a color I am familiar with when it comes to automobiles. Black is going to be a rich condition, white would be coolant, and blue for oil. MAYBE you are burning a combination of things. If it is constantly smoking you should see it in the plugs. Maybe take a temperature gun and check each runner, if you can see some sort of outlier with the temperature then you might be able to narrow the issue down to a particular cylinder?