The oil in your car pumps through the engine when it is in operation, protecting it from mechanical wear. The oil is sold in a variety of “weights,” usually expressed as xWy. The difference between 10W30 vs 10W40 oil is not very big, but to understand it, you will have to know basic facts about the viscosity of the fluids and the behavior of the oils under different temperatures.
Differences between motor oil: 10w30 vs 20w40
The numbers in a name such as 10W30 refer to the “weight” of the oil. It is a measure of its viscosity (i.e., how thick it is and how fast it moves). The lower weights mean the oil is diluted. And the higher weights mean that it is thicker. In a car engine, a more diluted oil is usually more useful at the beginning, when it has just started, and the oil should flow quickly through the engine. But when the engine heats up, a very diluted oil can be a problem.
The viscosity of a liquid such as motor oil varies with temperature. When heated, the oil becomes less viscous and flows more easily, thanks to the increased movement of the molecules that compose it. Since the oil passes through the engines and is therefore heated all the time, it will constantly change its viscosity, which must be taken into account when buying or categorizing.
The change in thickness with heat presents a problem. You want a diluted oil when the engine is cold, but you wait when it is hot, how do you find one that does not get diluted as the car moves? Motul oils, such as 10W30 and 10W40, contain long-chain polymers that expand and contract with temperature changes, altering the way oil behaves. In this way, it can be diluted at the beginning, but not so much at higher temperatures, as its original viscosity suggests.
In the measure of the weight of oil, the number before the W expresses the weight of the oil in the cold, and the second number its weight at temperatures above 100 degrees centigrade. The oils are still more diluted when they are hot than cold, but a 10W30, for example, will only be diluted to a weight of 30 while hot, while cold it will be a weight of 10.
10W30 vs 10W40
Both oils have the same cold weight, and it is a good weight, low enough for the oil to move through the engine quickly. A weight of 30 hot is very common and ideal for many engines, but if you have problems with wear or leakage, the 10W40 oil will provide more protection for a running engine, and will leak out more slowly.