I have only used Meguiars APC+, and clean, and steam(VX5000) through a white towel on a triangular brush, both sides, and I get them beautifully clean again.. Using Steam also makes them dry faster..
I steam them a little to get them damp, spray the APC+ on one side, (the side you see), brush it with the awesome Tuff-Shine Tire Brush, let it set for a minute, then steam through a towel that side.
Flip the belt over, repeat the process..
When done, I use another white towel to wipe them down and it is amazing how much brown body oil, etc., comes out on the towel.
I pull the belt all the way out - carefully - and to the very end, and then let a little go back in, and clamp it at the belt holder down there..
They all dry outside the vehicle, doors open, and it does not take too long, depending on your weather and temps, and they are good to go..
Same process of years, never had an issue, only had compliments from Clients, because they saw the different belt color and knew what I did..
And like everything else, once it is really clean, it smells really clean..
Dan F