There are five specific categories of base oils. These categories define the type of base stock the oil is formulated from. The categories are as follows. Note that the base oil group category is followed by the manufacturing method (in bold print) and then a description of the oil characteristics for each category.
Group I - Solvent Freezing: Group 1 base oils are the least refined of all the groups. They are usually a mix of different hydrocarbon chains with little or no uniformity. While some automotive oils on the market use Group I stocks, they are generally used in less demanding applications (hh).
Group II - Hydro processing and Refining: Group II base oils are common in mineral based motor oils currently available on the market. They have fair to good performance in lubricating properties such as volatility, oxidative stability and flash/fire points. They have only fair performance in areas such as pour point, cold crank viscosity and extreme pressure wear (hh).