This is just my own experience with regards to leading in my 686-3.
I used to buy commercial hard cast (I only assume they are really hard since I do not have a hardness tester) 158 grain LSWC. I loaded them over 3.7 grains 700X, which is a "regular" 38 special load. Definitely had to scrub my bore after two hundred rounds.
In the last 3 years, I have taken up casting using Wheelweights and Lee 158 grain SWC and RN TL molds, among other calibers. Lube, size them to .357, re-lube with Lee Alox or X-Lox. Loaded these bullets in 38 special cases, over 4.3 grains (lately 4.6 grains) 700X, which are in the +P or +P+ range. I have no leading or very minimal leading even after 500 rounds. The 4.3 grain loads have been chrono'd at 1,000 fps average out of my GP100 6 inch barrel.
With the latter loads, my GP100 does not show lead at all, if any.