Here is a very common concept:
“I used to use SAE15W-40, but dropped it in favor SAE 20W-50 for a better protection of my engine.”
But, is this really true?

The idea of using a more viscous oil (thicker) seems to have quickly emerged following the quick remedy offered to older engines, with a high rate of wear, which normally have a high consumption of engine oil. Switching from a more fluid oil (SAE15W-40 or SAE10W-30) to a thicker oil (SAE20W-50) may be a remedy that can save us an expansive repair. This is actually a delay. A SAE 20W-50 oil type has a thicker lubricating film that helps to separate metal parts, thus reducing wear.

Sounds good and is perfectly true. So oil should be as viscous (thick) as possible, isn’t it? Well not exactly so.
If oil is too viscous at ambient temperature it is very possible that it will not reach fast enough all engine’s surfaces, leading to faster and premature wear of the engine. Using high viscosity oil produces additional stress to the engine (the motion of the moving parts is more difficult in a more viscous ambient) and hence other shortcomings, such as: increased fuel consumption, and the additional engine stress, loss engine performance and in extreme cases even engine failures.
Some car manufacturers warned that the use of a very viscous oil could lead to engine damage. For example a SAE 20W-50 type oil used for General Motors LB4, L05 and L19 truck engines causes the so called “cold beat” phenomenon that seems to be a problem related to oil viscosity. Why? Because a SAE 5W-30 type removes this phenomenon.

The “viscosity” we have talked about before is actually a measure that tells us how “thin” or “thick” is a type of oil.

Bee honey, at low temperatures in January has a high viscosity and high resistance to flow (flows very slow). Water, on the other hand, at room temperature, has a low viscosity and low resistance to flow (flows easily).
Therefore, a low viscosity oil allows an easier start in cold weather. In addition, the more fluid oil (lower viscosity) will flow easier towards, and on engine’s components, and if well formulated, it will keep the viscosity even at high temperatures.

In summary, what would be the answer to the question “What kind of oil should I use for my car?” The simplest method to select the right oil for your engine is to comply with instructions given by the manufacturer in the user manual. There are loads of points of view regarding the right oil, of which, at least some of them, are acceptable, but always, it is wisest to choose the motor oil that meets manufacturer’s requirements.