The first three groups, are produced from base oils of refining crude oil. Base oil group III is the group which the highest process happens in it. The most expensive and highest performance in the mineral oil is derived from crude oil.

Group IV (basic PAO) is Synthetic oils, and chemically manufactured. Ethers, Glycols and other base oils which its specifications are not according to the first four groups aren't part of the group V of this classification.

Also oil that have low viscosity index due to the low percentage of paraffin in their composition, aren't part of this group.

Oily base oils are produced from crude oil and has a high percentage of paraffin material without oily (saturated ring). Only a few oil fields in the world can extract crude oil. Almost all oil fields in the Middle East have the nature of paraffinic and therefore base oils from refining Paraffin are based on paraffinic. Oily base oils due to lack Wicks (paraffin material) have very low pour points and thus in the formulation of oils for refrigeration systems are used. Oily base oils due to lack Wicks (paraffin material) have very low pour points and thus in the formulation of oils are used for refrigeration systems.

It is noteworthy that base oils of the group III produced by isomerization or severe Hydro cracking have very close properties to oils of synthetic group IV and lubricants producers can use the basis of oil products with the word "synthetic".

 All base oils, which are produced in Iran by solvent extraction and oil refining base. Obtained oil based, are placed in the Group I and classification of API. A unit for the production of base oils at group II and higher one has not been set up in Iran and lubricants producers supply need to base oils through imports.