From Motul Oils: Oil base Groups explained;
There are five different groups of base stocks used in engine oils.
Groups I and II are mineral oils (i.e. conventional petroleum oil), while Group III, IV and V are synthetics.
A Group III synthetic is more refined than mineral oil and typically hydro-cracked (higher pressure and heat) to achieve a purer base oil.
Group IV (PAO) is refined mineral oil that undergoes a special process called “synthesizing.” Generally speaking, Group IV performs better than Group III oils in handling heat, oxidation, low temperature start ups, and has a higher film strength and viscosity index (ability to flow). However, with today’s technology, some Group III oils perform as well as a Group IV.
Synthetic Group V (Esters) oils are mostly made from vegetables, minerals and animal fatty acids. Esters are much more expensive because the ingredients are collected from nature, then get synthesized (which is a very expensive process).
Group V Esters have all the advantages of a Group IV PAO, plus they can handle even higher temperatures. When Esters are burned, they leave far less coking deposits and are attracted to metal parts with an electro-chemical bond five times stronger than mineral oil.