Oils are chosen by the manufacturer to give the right thickness at the normal operating temperature of the engine.
The low-temperature viscosity (the first number, 5W in a 5W-30 oil) indicates how quickly an engine will crank in winter and how well the oil will flow to lubricate critical engine parts at low temperatures. The lower the number the more easily the engine will start in cold weather.
The high-temperature viscosity (the second number, 30 in a 5W-30 oil) provides thickness, or body, for good lubrication at operating temperatures.
Many think that the “W” in 10W-30 means “winter”.
The W is just a designation of one type of testing vs another.
grades denoted with the letter “W” are intended for use in applications operating in low-temperature conditions. The “W” was originally coined for lubricants that were considered “winter grade.” Today, these products are formally called multigrade lubricants, whereas the grades without a “W” are recognized as monograde, or straight grade, lubricants.
Two series of viscosity grades those containing the letter W and those without. Single viscosity grade oils with the letter W are defined by maximum low temperature cranking and pumping viscosities and a minimum kinematic viscosity at 100°C.
Single grade oils without the letter W are based on a set of minimum and maximum kinematic viscosities at 100°C and a minimum high shear rate viscosity at 150°C.
A 10W-30 synthetic oil is based on a 30 grade oil. This is unlike the counterpart mineral oil based on a 10 grade oil. There is no VI improver needed. The oil is already correct for the normal operating temperature of 212°F. It has a thickness of 10 while you drive to work. It will never thin yet has the same long term problem as the mineral based oil. They both thicken with extended age.
You do not need to use the exact oil type and brand that your car manual tells you to use. Oils are pretty general. They are not that different.
The best way to figure out what viscosity of oil you need is to drive the car in the conditions you will use. Then use the oil viscosity that gives you 10 PSI per 1,000 RPM under those circumstances. For some reason very few people are able to get this simple principal correct.
These same rules apply to engines of any age, loose or tight. Just because your engine is old does not mean it needs a thicker oil. It will need a thicker oil only if it is overly worn, whether new or old. Yet the same principals of 10 PSI per 1,000 RPM still apply. In all cases you need to try different grade oils and see what happens. Then choose the correct viscosity.
Some people have tried this and occasionally get a somewhat low oil pressure while at idle. This is fine. There is no stress on parts at idle, the smallest oil flow will do the trick. It is at higher RPM where more BHP is produced. This is where we need the flow.
Motor Oil 101 - Bob is the Oil Guy
Look for the Products that Carry the API Quality Marks
The 0W-20 may provide you an edge in fuel economy. If you have a finely tuned ear you may hear lifter tick on start-up which would quickly disappear