Mineral oils are refined from petroleum, but even mineral oils contain some synthetic compounds or additives to improve them. Semi-synthetic oils are a blend of mineral and synthetic oils. They have definite improvements over pure mineral oils. Semi-synthetics can contain “hydrocracked” bases. Hydrocracked oils are mineral oils that have been subjected to intense pressure and temperature to change the structure of the molecules, making the resultant oil more stable and resistant to evaporation at higher temperatures. Semi-synthetic oils don’t cost much more than mineral oils do, but offer advantages over the latter.