Engine oil has a limited life. It can be affected by dust, metallic shavings, condensation and antifreeze. After a certain point, engine oil carbonises and loses its lubricating qualities. Once that happens, the engine gets contaminated with carbon deposits or sludge. When you change oil at or before the manufacturer’s suggested interval, the oil doesn’t get a chance to carbonise, and the engine is thus kept clean. The oil filter should be changed regularly too.

If engine oil is not changed, carbon deposits will clog the oil pick-up screen, decreasing oil supply and increasing friction, and wearing out the engine much faster.

Always use the oil recommended by your car’s manufacturer. Oil grades have changed over the years, so make sure you use the right one. Follow the guidelines provided in the vehicle owner's manual for the recommended change intervals.

If you don't remember when you last changed the oil, just check with the dipstick. Do this at least once a month – more often if the car has chalked up a high mileage.

Check under the car for leaks. If you find any, fix them before they result in more serious problems.

Mineral oils are produced by distilling and further refinement of existing crude oil stock.
Most synthetic motor oils are derived from mineral oil that is chemically altered in the lab. Semi-synthetic oil is a blend between the two.

Synthetic oils, pioneered in the 1970s, are now sold by most major oil companies. These oils aren’t strictly artificial – they still have the same natural ingredients found in mineral oil, but those ingredients are recombined to yield synthesised-hydrocarbon molecular chains with desirable characteristics not found in even the highest-quality mineral oils.

Synthetic oil has no contaminants, and its molecules are more consistent in size and shape. It is therefore better able to withstand extreme engine temperatures and can work longer without losing its lubricating qualities.

By contrast, mineral oils contain small amounts of sulphur, wax, and asphaltic material that can promote sludge build-up. Unstable molecules can also vaporise easily or oxidise in extreme heat.

The main disadvantage of synthetic oils is that they cost significantly more than mineral oils.