Synthetic oils are chemically synthesised to meet the exacting demands of modern engines. This makes them more expensive to produce, but because they are tailor-made they provide the best performance, protection and fuel economy. They also remain stable at very high temperatures and fluid at very low temperatures.
Semi synthetic oils are a blend of mineral and synthetic oils. They provide better performance, protection and fuel economy than mineral oils, but not as good as a full synthetic.
Mineral oils are made from naturally occurring crude oil which is refined and processed to remove waxes and impurities. Mineral oils tends to be found in the thicker viscosity grade of engine oils.
The reason why synthetic oils outperform minerals oils is down to their molecular structure.
Mineral oil is made from a complex mix of molecules of different shapes and sizes. In contrast, synthetic oil is created in a carefully controlled process and tailor-made to meet a more exacting specification. The result is:
Better lubrication at both hot and cold temperatures