I spoke with a man who served on a submarine during WW2, and was basically the boat's armorer. He was in charge of caring for the web and leather gear as well as the firearms. While in port, the subs posted a 24/7 security detail in all but the worst weather, so the web and leather gear got plenty of seawater and salt on it. His solution was to thoroughly rinse belts, holsters, slings, etc, in fresh water and apply neatsfoot oil while the leather was still damp. He'd follow that with saddle soap and black shoe polish.
Historically, I think most most holsters weren't cared for much at all. How many original rigs do you see out there? Neatsfoot oil is a good answer for a while, but it eventually reacts with the oils found naturally in the leather and causes the fibers to break down, causing cracking. Saddle soap is a fair preservative, but far from perfect. I like mink oil, but I wonder how available it might have been to most folks in 19th/early 20th centuries. Today, I'm all about Sno-Seal, Lexol, and atom wax.
If you really want a tutorial on keeping leather alive, go to a saddle shop and find out what the horse folks are using on their $2000 saddles. Horse sweat is just about as bad for leather as saltwater immersion.
I don't imagine Roy was wearing his "parade" rig when he jumped into the surf. It was most certainly a stunt rig, and likely worn by a stunt MAN. I have seen a big ol' buscadero rig that was painted black, with painted-on "silver" spots. I'm willing to bet it was a stunt rig. Crash Corrigan had one that was so loaded down with silver that it weighed over 35 pounds, and some of Roy and Gene and Hoppy and Clayton's leather couldn't have been far behind.