Contrary to popular belief the oil pressure gauge does not measure the amount of oil in the oil pan or sump of the engine, but rather the pressure at which the oil is being forced around the engine. Why is the oil pressure so important? Does it matter if the oil is moving, as long as it is covering the parts to lubricate and prevent wear and tear from friction? Yes, because the thickness of oil covering the parts is important and the oil is used to reduce heat too.
The oil pressure sensor measures the pressure of the oil, but what factors affect the pressure? Viscosity, or ‘thickness’, is one. The oil needs to be thick enough to provide a layer or film of protection over the moving parts of the engine yet not too think so that the flow of oil around the engine is reduced. The temperature of the oil affects the viscosity.  When the car is cold, before the engines has been running for a while and heated up, the engine oil will be more viscous or thicker and the oil pressure sensor will read a higher value (although in most cars you do not see the actual pressure being measured by the oil pressure tester, the light on the instrument panel only comes on when the pressure is too low).  Once the engine has heated up the oil ‘thins out’ as the viscosity reduces. If the engine is on and the car is idling this also lowers the oil pressure. The pressure of oil when the car is hot and standing still is the ‘worst case’ for a car and manufacturers specify the lowest allowable pressure under these conditions.

Types of oil

The viscosity of engine oil is generally specified as a number, letter, number on the container, Like this:

5 w 40

The first number refers to the viscosity of oil in the engine when cold. The W stands for ‘winter’.  Eg 5w.  Lower ‘w’ numbered oils are usually better in the cold and more efficient. The second number is the viscosity rating at normal engine operating temperatures.
Your manufacturer’s handbook will specify acceptable type of oil to use. Some car manufacturers produce their own oils specially formulated to protect their engines. Volvo, Audi and BMW for example. Using the manufacture’s oil can prolong the life of the engine by providing better protection.