AN oil change is a critical part of the car maintenance process. However, in-between a full and scheduled change, topping up engine oil will become necessary as the oil level drops. But be careful, do not add oil without informed assistance, using the incorrect oil in your engine could quite likely result in serious and expensive damage.

If you look on shelves in auto parts stores and service stations, you'll see engine oils labelled for all kinds of specific purposes — high-performance engines, new cars, higher-mileage vehicles, heavy-duty/off-road SUVs. This can make choosing an oil a little confusing.

"Oil manufacturers regularly put out new standards. It is very hi-tech these days," said David Bell of ATL Racing, the team behind Doug Gore's high-performance Audi TT.

"There are natural oils and synthetic oils. Diesel vehicles and gas vehicles use slightly different oils. And there's an oil for normally aspirated diesel engines and another type for turbo," Bell shared.

He added that synthetic oils, while higher priced, are much more resilient and last much longer.

With all that's available, one should also be extremely careful that the correct oil viscosity is used in the engine. Designations on the label — such as 10W30 or 20W50 — tell the oil's viscosity.

In the owner's manual, car manufacturers recommend oils for the brand new vehicle, and 10W30 is commonly used in new engines instead of heavier oils. But what's really important is that you use the oil viscosity the owner's manual recommends.

Viscosity — a fluid's resistance to flow — is rated at 0° F (represented by the number preceding the 'W' and at 212° F (represented by the second number in the viscosity designation).

Motor oil thins as it heats and thickens as it cools.

So, 10W30 oil has less viscosity (flows easier) when cold and hot than does 20W50.

Once the engine is running, the oil heats up. The second number in the viscosity rating — the '40' in 10W40, for example — tells that the oil will stay thicker at high temperatures than one with a lower second number — the '30' in 10W30, for instance.

The more resistant the oil is to thinning, the higher the second number.