Everyone seems to have their own favorite process for preserving old leather, but applying almost any preservative to it is almost certainly the wrong thing to do. Here's why.
Pecard's, neat's foot oil, silicone greases and the like will actually accelerate the long-term deterioration process. These are all designed to soften, preserve, and waterproof footwear that isn't meant to last for more than a few years.
Leather is simply dead animal skin, after all. Given time, it will rot, and nothing can totally arrest the process. The rate at which the item rots has much to do with the quality of the original tanning process, and so cannot really be predicted. Really old leather will simply not last forever, regardless of what's done to preserve it. That's why we see so few leather items from the 19th century and before. We have few or no leather items from medieval and Roman times (other than volumes on vellum) and this is why.
If one is really concerned, one should keep leather away from moisture (including humidity) and sunlight. Loosely rigging that original sling to your collectable rifle in a climate controlled safe is a good option. I have other leather items stored in sealed plastic freezer bags in dark cabinets. A small dessicant packet in each bag helps preserve metal parts as well.
I have one of the unissued Portuguese M1937As (98Ks) that were brought into the country a few years back. These had been stored in sealed crates since leaving the Mauser factory in 1941. Mine still has the original sling rigged to it. It's never been removed from the gun, and is in original new condition, as is the rifle. Water proof, light proof storage preserved both.
Almost regardless of what we do, these leather objects will be rotted away in a couple of centuries, or so desicated that they cannot be handled without falling apart. So, enjoy them today. With the proper care, your prized WWII rifle will probably survive, but associated leather items will most likely not.
Article for your review and discussion:
Conventional wisdom with museum conservation, and particularly among collectors, seems to change from time to time. I've heard that Pecards is used by the Smithsonian and other museums, but I've never seen museum conservators state that. Probably internet lore.
I conserve paintings, not leather, but I have talked to museum conservators who work with leather and according to them, treating with Pecards or other store-bought conditioners will only serve to accelerate the destruction of the leather. It is my understanding that those conditioners/dressings can attract more airborne contaminants than untreated leather, and interact unfavorably with leather in the long-term.
The stability of leather greatly depends on the tanning process used, plus environmental conditions. I have leather in my collection that is 230+ years old. It has never been treated with any conditioners – partly the reason it still exists. Coating it with a commercial product is not something I would ever consider.
Leather conservators use consolidants such as Klucel G to conserve fragile leather, but never commercial leather treatment products. Klucel G is used because of its ability to be dissolved in alcohol, a solvent that minimizes the darkening of leather (as opposed to water). Treatment is kept to an absolute minimum. You don’t coat the whole object. For example, a conservator might use a consolidant on just the part of a leather sling where it is damaged, such as where it wraps around the sling loop, and leave the rest of the leather alone.
If leather has been poorly processed, it’s not going to hold up well no matter what, so you’re going to have to live with degradation. Chances are the leather you are trying to preserve has already been made worse because some collector before you coated it with some type of store-bought conditioner. It is getting harder to find leather that has not had these conditioners applied to them, and I really dislike the darkening that occurs with these products. There are other conservation methods available.
It is important that Hambone pointed out that the article promoting the use of conditioners was written by someone who benefits from the sale of Pecards. If you look at any articles that promote leather treatments, they are generally written by collectors and not by people with advanced training such as conservation scientists. Bob in Ohio has the right idea – leave things alone, particularly if they have never been treated before.
Mike