Liquid Viscosity is bascally how runny/thick it is.
High Viscosity means thicker
Low Viscosity means thinner
Thicker oil, more resistance, engine works harder
Thinner oil, less resistance, more power
Thicker oil is only used in BIG HP cars and 70-80s cars for extra protection.
Thinner is for High Revving 4cyl.
First number means viscosity when COLD
Second number means Viscosity when HOT
(Oil normally becomes thinner as it gets hot, but dont forget it has additives to counter that)
All explained by Castrol
Thats why I think, nothing is better than 0W-40 or 0W-30. With these oils, you can drive off without having the warm up your car, because of its thin state when cold.
So guys, you dont need to think too much of the brand, when viscosity is the most important
YouTube - Tech Talk on Engine Oil - with Castrol