The mineral oil contains sulfur.  When moisture arrives in the system and it's all heated the product is sulfuric acid, which is not a real good thing.   Probably also not going to be the end of the world....the truck we changed last week has had mineral oil in it for at least 5 years since the last change.   It had mineral oil because at the time it seemed to expensive to replace with synthetic.  Synthetic is quite a bit less expensive then it was. 

If the axle has synthetic in it now the change interval is something like 1,000,000 miles and time is not stated.  That's my justification for putting synthetic back in....we shouldn't ever have to change it again, so the cost isn't all that big a deal.