Originally Posted by dbhdnhdbh
I am NOT a leather or shoe expert, but from what I have read on the subject, I get the following combination of fact and speculation.
Fact: Water is an integral part of the structure of leather. If the water content drops too low, then the leather suffers permanent damage, and this can lead to cracking. Leather dries out over time, so it does need water restored to it. The rate of this drying, of course, depends on the humidity under which it is stored, and the composition of the leather.
Fact: Most water containing conditioners also contain humectants (as far as I can tell). Fat liquoring compounds certainly contain them. These help hold the water in the leather, but eventually may leak out or break down. Once the humectant goes, the leather is less able to retain water from the atmosphere or sweat, and the leather becomes even more dry. Conditioning with something like Lexol, PROBABLY adds humectants as well as water.
Fact: Oils and waxes are a necessary part of the fat liquoring of leather, and are also needed to maintain strength and flexibility. Modern tanning methods carefully control this process and the fats are permanently bound to the leather, hence they do not need to be restored.
Speculation: It is not clear that these "modern tanning methods" apply to the traditional process used to manufacture shell cordovan. I could not find anyone who addressed this, but the methods Horween demonstrates are wholly different from what is described for the current approach. It is possible that shell undergoes modern fat liquoring, but it does not have the romantic cachet of the open vats, so they don't show it. If shell does not get fat liquored with this chemically-controlled method, then perhaps it can lose enough fat that it may need to be restored. Nick Horween is on record discouraging most attempts to do this, so it may be that shell is made differently, but still with so much wax that there is no need to add any.
Speculation: I could not find more than a casual side comment as to how long the current high tech approach to fat liquoring has been widely used. Someone suggested several decades, but that is the closest estimate I could find. If you collect vintage shoes, they may not have been made this way, and perhaps they may need the fat restored.
SPECULATION: Based on the above, I can believe that shoes treated with oils and fats, but no water containing conditioner, could dry out and crack. I could believe this might happen particularly quickly if they are stored in a low humidity environment. A heavily air conditioned room would be bad, an unconditioned space in a warm, arid part of the country would be even worse. This would not mean that the Renovateur, etc, caused the cracking, but by failing to restore water, they may have failed to prevent it.
Since I buy old, used shoes, and bring them up to my low standards to wear, I do not assume they were made with modern methods. They typically appear to have gone many years without conditioning, and seem quite dry. I use Lexol, multiple applications over several days, to restore moisture. After that, some still appear "dry" on the surface. I find that Renovateur, by adding oil, refreshes the appearance of the surface, and gives them the light sheen I seek. I don't rely on Renovateur to serve as a maintenance conditioner, since I don't think it does the job of adding water (Of course, I could be wrong about this). Lexol is also vastly cheaper. I use Venetian to add some shine to the surface if I want that, and following Nick Horween's suggestion, that is what I use on shell.
The shoe experts DWFII, Cobblestone, and Nick V have all said they are long time satisfied users of Lexol, which is a more meaningful and practical endorsement than all the pseudo science above.