Inside the engine oil is needed to provide cooling, cleaning and lubrication. To successfully perform these three jobs it needs to be pressurized. And by this we mean it has to be moved around the engine quickly and efficiently. If the oil pressure falls, as indicated by the oil pressure gauge it cannot do these things as efficiently as required. Some of the things that cause low oil pressure are
Leaks
Some leaks are easy to spot. They leave tell-tale signs such as stains on the pavement. But the oil can also leak inside the engine. If oil is seeping away into the coolant it will turn it brown/black. If oil is entering the combustion chamber where it is being burnt along with the gas and air mix and then you will see a hint of blue in the smoke coming out of the exhaust. This problem is caused by leaky gaskets around the Pistons. With age the gaskets wear away or become brittle and cracked. This can be difficult to fix because of their location deep inside the engine and this in turn can make it an expensive problem to solve.
Wear and tear
The older a car gets the more worn its engine becomes. Even tiny levels of wear and tear can change the amount of space inside the engine. If the oil has to flow through a larger area it’s pressure will fall. This can quickly become a vicious circle. Depleted oil pressure can increase the damage caused by moving parts which are not sufficiently lubricated, this in turn decreases the oil pressure etc. Blockages and the buildup of sludge inside the engine also inhibit the pressure. This is why oil also contains detergents to clean engine.
A good resource to learn more about the oil system in your car is the oil pressure gauge guide blog.