USING a poor quality engine oil can shorten the average car's engine life by as much as 60,000 miles, says the AA.

However, with a dozen options on display in some accessory shops, choosing the most suitable lubricant can be confusing.

Today's high-specification lubricants have other important functions to help extend the engine life, such as the reduction of piston ring deposits, control of exhaust emissions and reduction of metal corrosion.

There are two main grading systems which indicate the performance and properties of today's high-specification engine oils: the API (American Petroleum Institute), and the European ACEA.

As well as a performance grade, there is also a viscosity rating, indicated by a series of numbers which show the thickness of the oil when hot and when cold, for example 0W/40 or 20W/50. The first number indicates how thick the oil is when it's cold and the second gives its thickness when hot. The '0' does not mean that it has no viscosity and all oils are thicker when cold than when hot. However, the lower 'W' numbers help with cold starting and the higher numbers imply more body. Most current oils are referred to as multigrade oils which means that they thin down less than older monograde oils. By itself, the viscosity grade does not indicate an oil's quality.

Tips for choosing the right oil:

• Check the engine manufacturer's specification - this is usually in the car handbook, or available for reference in the better stores.

• Don't worry about using an oil grade higher than the minimum specified, as it isn't a problem for the engine. So if the handbook says API SH, an SL oil can be used, but it is likely to be more expensive on the pocket.

• If use is severe, such as high speed journeys or towing, then a higher grade may be worth paying for.

• Note that warranty conditions may specify a lubricant.

• An older engine, starting to burn oil, may be better with a lubricant with heavier viscosity.