To understand motor oil ratings you should also understand how viscosity works. Motor oil is graded by its level of viscosity as well as its performance. When motor oil is graded according to its viscosity it is graded by how well it can be poured under certain conditions.
What does the W mean? Good question. The W refers to Winter and how well the oil copes with winter pouring and performance. Thinner and low grade oils like 5W or 10W are usually far more suited to lawnmowers and air cooled, small vehicles, but not the car motor oil type.
Oil is measured largely by its weight, which depends on its thickness. Thinner oil has a lesser weight rating than thicker oil, for example. Single oil viscosity is not as common as it used to be, owing to the heightened technology now used in more modern vehicles. Instead, the use of multi-grade oils is employed for better vehicle performance. A motor oil ratings chart will give you a better idea of how oils are graded, but suffice to say that 20W-50 multi-grade oil is the better performance oil. It sits in the oil ratings chart as one of the thickest and most heavy of motor oils. The 20W signifies the average Fahrenheit temperature that the oil can cope with. In this case it is 20 degrees Fahrenheit, but it can cope with anything down to zero.
Usually an engine with an overhead cam needs a thinner oil, somewhere between 5W-30 and 5W-20. This assists with lubricating the overhead cam on a cold start. With older engines you might experience an increase in the bearing clearance and in this situation you would be advised to switch to a thicker engine oil. Cars with higher mileage may be more likely to experience this, so increase the W factor by 5.
Most motor oil has added polymers which prevent thinning of the oil at higher temperatures. This effectively adds to the motor oil weight by thickening it sufficiently that it doesn’t weaken during performance. Dispersants and detergents are also added to help keep the oil mobile and clean. Corrosion inhibitors are also part of the ingredients to protect the inside of the engine from rusting and oxidation.
The Society of Automotive Engineers (SAE) is largely responsible for the grading of motor oil. It first decided on a numerical code to describe and identify grades of oil. The grades start at 0 and go through to 60. From 0 it goes up in increments of 5 until 25, when it then increases by 10, until it gets to 60. All numbers up to 25 will have a W rating, but 20 can either be with or without one. Manual transmission, axle and gear oils have a different grading, so don’t make the mistake of confusing them with engine grade oils.