There is some question as to whether or not mineral oil should be used on shoe/boot leather. I can tell you that you should avoid using mineral oil on your shoe and boot leather if you can.
The reason I say “if you can” is because a number of leather conditioners and leather protectors include mineral oil. Fortunately they will typically state something along the lines of “Contains petroleum by-products” or “Contains petroleum distillates” somewhere on the label. Some tanneries also use mineral oil in their fatliquoring process.
Mineral oil is a basic straight chain hydrocarbon and comes is various grades defined by the carbon atom count, which ranges from 18 to 30 carbon atoms. Mineral oil used in shoe products has a molecular composition of about C25H52 (25 Carbon, 52 Hydrogen). Mineral oil is also a non-polar hydrocarbon and as such cannot have a pH value.
Mineral oil is also extremely hydrophobic because it is a non-polar substance, and the molecular structure has fewer reactive tertiary hydrogen and carbon atoms which minimizes direct reaction with oxygen. What this means is that mineral oil is much more of a water barrier than triglyceride based oils like animal and vegetable fats. In fact mineral oil is considered to be occlusive (a stop to moisture), whereas triglyceride based oils are considered semi-occlusive (allowing your shoes to pass perspiration for evaporation to some degree). Moisture, trapped in the leather by mineral oil, can lead to dry rot of the leather over time.
Another aspect of mineral oil is how it oxidizes: All oils oxidize over time, even mineral oil. Oxidation at the molecular level can happen in one of two ways: the gain of oxygen atoms, or the loss of hydrogen atoms. Because triglycerides already involve oxygen bonding in their molecular makeup they oxidize through increased oxygen. Since mineral oil can’t really bond with oxygen (for the same reasons it is hydrophobic) it must oxidize through the loss of hydrogen (as a side note: hydrocarbon molecules can be oxidized with oxygen atoms, but only at very high temperatures).
This loss of hydrogen atoms begins breaking down the molecular composition of the mineral oil (originally C25H52) and as it breaks down it becomes more volatile (the molecular composition of naphtha [the petroleum by-product used as a solvent in shoe polish –and other things] can be as big as C12H26). The smaller the size of a hydrocarbon molecule the more volatile (and toxic) it becomes. This is the reason mineral oil is never used to maintain old leather in museums and such.
So why is mineral oil used in shoe/boot care products and in fatliquoring? Mineral oil is used because it is very cheap compared to triglyceride based oils, and it provides a stronger moisture barrier.
Unfortunately, if you plan to keep your shoes/boots longer than a few years, the mineral oil will do more harm than good.