From Moebius’. and Otto Frei:
Nature watch oil is a mixture of neatsfoot and mineral oil. Nature oil has and is still quite satisfactory except that after a period of time in contact of metal, they change chemically and form a gummy substance. This was great for watch makers since they would get to make money cleaning and oiling the watch every 18 months or so. After WWII in the 1950s the first Synthetic Oils came on the market. Most watchmakers did not rush to use these oils since they were bad for business now that the watch serviced with Synthetic oil would not come back to him for service for another 5-7 years he would lose a lot of income. Synthetic oils have the same lubricating properties as natural oils, however they will not change over time, and will not gum up, cause corrosion and become acidic it left in a watch for many years without a cleaning of the movement.
I’ve cleaned many watches that were serviced w/ natural oils, and they can cause more challenges when it comes to cleaning. Some where almost like a coating of varnish in the jewel cups
Hope this helps