Choosing the right oil: There are three kinds of oils - mineral, synthetic and vegetable (castor oil was

once used in racing car engines.) All premium grades of engine oil contain additives in some form of the other to protect against oxidation and froth given property of oils to deteriorate. Some additives coat internal moving parts like pistons and cylinder wall with a thin protective layer to provide boundary lubrication, while others prevent formation of hard lacquers which shorten engine life.

Viscosity or thickness of oil is the property of an oil to separate moving parts of an engine and seal off hot, high-pressure gases, Oils which are thick seal off the gases but prevent moving parts from sliding easily. This creates engine drag and increases fuel consumption. Too thin oils could break down permitting contact between moving parts and increasing wear.

Viscosity of oil is identified by its SAE number (Society of Automotive Engineers which devised these standards.) The numbers SAE20, 30, 40 and 50 indicate that an oil falls within certain viscosity limits at 99deg.C, whersas the numbers SAE 5W, 10W indicate oils that fall within certain limits at -18deg.C. The lower its index the thinner the oil. A multigrade oil SAE 10W/30, 20W/40 is a lube that has qualities of high viscosity index. Its advantage is it ensures easy cold starts in winter yet retains its qualities at higher temperatures.

Corrosion reduction is another function of oil to prevent corrosion caused by formation of acids during combustion. An alkaline additive in oil neutralises the acids.

Detergents and Dispersants: Partially burnt acids and such and sooty, tarry substances find their way past piston rings into the sump. If not absorbed by the oil they form deposits in piston ring grooves, on filters and oil ways obstructing oil circulation and leading to piston ring seizure. An oil with detergent and disperant additives holds these impurities in suspension so they can be absorbed by the oil.

Gear oils are generally the same as engine oils unless specified otherwise by the manufacturer. Certain synthetic oils contain extreme pressure additives which act when contact pressures and temperatures are too high for normal mineral oils.

Automatic transmission oils have to meet exacting specifications to ensure that functions of the torque convertor, clutches and brake bands are smooth and jerk free. The working temperature of these oils is generally very high so special blends of mineral oils and additives are required.

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