Everyone has an opinion on leather saddles and up to now I had promised myself I wouldn't comment on this subject...however...here goes.

In the "forming" process which shapes these saddles the natural oils are leached out of the leather...this is the reason that the saddle is so hard when new. Having no natural oils in the leather does allow the "breaking-in" process to happen as the leather fibres are now brittle which means that they can now break making the depressions caused by the rider's sit-bones.

The problem is that as they leather ages, being without the natural oils, it eventually crumbles and develops cracks. It is possible to slow/stop this by replacing the original oils with some product but whatever used should not over-soften the leather making the saddle useless. Proofide, at one time vetoed by Brooks from being applied to the underside of their saddles is now approved by them for this purpose....wrongly in my view as this will eventually over-soften the saddle. The reversal of the original advice only came about when the new company realised that their saddles in the US were ridden by riders who did not use mudguards.

As to a suitable product, again everyone has an answer and even Sheldon got this one wrong when he advised Neatsfoot oil.
Neatsfoot oil is organic in nature and attracts various types of fungus over time so should not be used. A modern product which may slightly soften (used with care) but which waterproofs and preserves is the answer, and today those working with leather would probably recommend "Hydrophane" or something very similar.