When is it appropriate to use a synthetic oil? What is it important to know about synthetic oils?

Unlike "mineral" oils which are manufactured by distilling crude oil and then subjecting it to special processing, "synthetic" oils are manufactured in the chemicals sectors by adding together identical molecules in order to obtain oils of varying levels of viscosity.
The main synthetic oils are synthetic esters that are often used to create biodegradable lubricants, polyalphaolefins (whose chemical composition makes them very similar to mineral oils) and polyalkylene (glycols which have improved lubricating capacities).

The main advantages of mineral oils – apart from the special benefits derived from their molecular composition (biodegradability for synthetic esters, for example) – include a better ability to withstand ageing (meaning a longer service life) and the fact that they can be used over wider temperature ranges, giving them more multigrade properties than traditional mineral oils.