What is the difference between transformer mineral oil vs FR3 oil?

Mineral oil is petroleum based, widely used, and cost effective, but it has lower fire point than many ester fluids. FR3 is a natural ester fluid marketed for higher fire safety and biodegradability, often improving environmental risk profile and allowing different siting options. Esters also behave differently with moisture, tending to hold more water in the fluid while keeping paper drier under some conditions. Selection depends on performance targets, standards, and life cycle economics.