I have had mine installed for about 2 or 3 months now daily use and yeah the very outer silver lining of the turtle turns a brass/copper gold color but honestly I almost like the different color on there because it brings out the turtle in the middle more. Almost adds that classy gold and silver watch look to the knob itself.
Honestly other than that I would assume that a light luke warm soap and water mix would work. I wouldn't use a soap with a bunch of harsh chemicals but invest into like a good organic soap or something along those lines if you want to be super safe.
I wouldn't ever clear coat it though.. My car gets way too hot and sits in direct sun all the time and would come out one day during lunch and find a melted clear coat mess on the leather boot.. so in conclusion
I AGREE ON HOW PUTTING A PROTECTIVE COATING ON OUR KNOBS TAKES AWAY ALL THE FEELING AND FUN...
Just gotta get her ready and keep going over and over until it breaks. Then will have to take a break and clean her up and then do it all over again