Just for clarity, USFA's bone case and Turnbull's bone case work is the real deal, and the same thing as Turnbull does USFAs bone case work. I had parts done by Turnbull and had a chance to ask about that very warning. After the parts are truly bone charcoaled and quenched in water they are case hardened not just case colored like a chemical process, which leaves those beautiful colors that when seen, anyone can tell the difference between the real deal compared to the chemical finishes used by other gun makers. I have a Cimarron with what they call a USA finish, and it was sent to Turnbull for real bone case hardening. Afterwards, Turnbull puts on a clear coat to help protect the coloring from fading over time. Bone case hardening's coloring will fade from use, sunlight, and cleaning because of the direct handling. That clear coat is an attempt to protect the coloring for as long as possible. The use of certain strong chemicals will soften that clear coat over time and make the clear coat lose its ability to protect the bone case hardening's coloring, not the hardening. The hardening process would only be hurt by heat.
With all of that being said, I personally use Break free and shooter's choice because thay are milder cleaners
that will not hurt the finish, but being careful when cleaning is always wise when trying to protect the beautifull finishes these USFA sixguns have.