What matters most is the carmaker's oil specification, given in the service book or owner's manual, for example VW 504 00/507 00, BMW Longlife-04, MB 229.51 or ACEA C3. An oil fits when the same specification appears in its name or description. Choosing by viscosity alone can get you an oil that does not meet your engine's requirements.
Viscosity, such as 5W-30, has two parts. The number before the W shows how well the oil flows in the cold: 0W and 5W suit an Estonian winter well. The second number shows how thick the oil is at operating temperature. Use the viscosity given in the service book.
The amount an oil change takes, filter included, is also in the owner's manual. A 5 litre can is enough for most cars, and what is left is for topping up. Choose gear oil, coolant and brake fluid the same way, by the carmaker's specification, not by colour or price.
