There are many facets to this question which prevent a simple answer. To begin with, synthetics as a class whether it would be PAOs or Polyesters tend to be more hydroscopic (i.e., water absorbing) than mineral oils. That is because their organic structure tends to make them be more polar which would create the ability for their absorbing greater levels of atmospheric moisture than petroleum oils. You now need to factor in the additive packages as some of these tend to enhance this absorption of water, and as we all know there are many different additive packages that are used. Tha absorption of moisture caused by the normal "breathing" of systems when place is a storage environment can do some considerable damage due to the generation of rust and system corrosion. Because of this, the Military that has lots of vehicles and equipment that are not always in active use requires an engine oil to be formulated with rust inhibitors to protect against potential corrosion problems that would occur if one were to use conventional oils that are formulated for SJ/SL/etc. service. There is also the problem that absorbed moisture can react with the individual additive ingredients and render them not as effective as they should. Again , we are talking about storage periods starting from 6 months and beyond. As a matter of record, many of the manufacturers of farm machinery (Deere, Massey Ferguson, etc.) have specifications for their engine oils that follow the exact same practice of the military.