The American Petroleum Institute (API) developed a classification or grouping of lubricant base stocks to better show the differences between the different types. Each type of lubricant base stock is included in one of five groups as summarized below: Note: on Bitog group III has been the synthetic baseline:
Group I
< 90
> 0.03
80 ≤ VI < 120
Conventional Petroleum Oil
Group II
≥ 90
< 0.03
80 ≤ VI < 120
Hydrocracked III
≥ 90
< 0.03
≥ 120
Hydrocracked and isomerized IV
Polyalphaolefins (PAOs)
Group V
All other stocks not included in Group I, II, III or IV. (Esters, PAGs, silicones, etc.)
In the API classification, all the oils except Group I can legally be called “synthetic”, but there are different chemical properties and characteristics associated with each of Groups II through V.
Group I
< 90
> 0.03
80 ≤ VI < 120
Conventional Petroleum Oil
Group II
≥ 90
< 0.03
80 ≤ VI < 120
Hydrocracked III
≥ 90
< 0.03
≥ 120
Hydrocracked and isomerized IV
Polyalphaolefins (PAOs)
Group V
All other stocks not included in Group I, II, III or IV. (Esters, PAGs, silicones, etc.)
In the API classification, all the oils except Group I can legally be called “synthetic”, but there are different chemical properties and characteristics associated with each of Groups II through V.