Lubricating oils are further classified as follows.

1. Animal & Vegetable Oils:

• Before the invention of the petroleum industries, they were the most commonly used lubricants.
• They have good oiliness. However they are expensive. Undergo Oxidation easily, forming gummy and acidic product and get thickened on coming with contact with air.
• They have some tendency to hydrolyze when allowed to remain in contact with moist air or aqueous medium, so at present they are rarely used as such,

2. Mineral of Petroleum Oils:

They are obtained by distillation of petroleum. The length of hydrocarbon chain is varies between 12 to 50 carbon atoms. The shorter the chain oil has lower viscosity than the long chain oils. These are most widely used because they are

• Cheap,

• Available in abundance,

• Quite stable under service conditions.

Impurities in Crude petroleum can be removed by dewaxing, acid refining & solvent refining.

3.Blended Oils :

• No single oil serves as the most satisfactory lubricant for many of the modern machineries. Typical properties of petroleum oils can be improved by using specific additives, these are called Blended Oils. Fatty acid, organic compound, glycol.
• It is done to reduce pour point, improve viscosity increase oiliness, resist oxidation, reduce corrosion & improve colour.

4.Synthetic Lubricating Oils :

• There are chemically prepared compounds. Where mineral oil fail to work, synthetic oils are used. They are very effective in severe condition and are used in jet engines, rocket motors, submarines, etc.
• They are very expensive and sometimes they are used by mixing them with mineral oils.