The Society of Automotive Engineers (SAE) developed a grading system over 100 years ago to classify the thickness of an oil by viscosity. It has been refined over the years but is still used by all oil and automotive manufacturers globally.
Very crudely, an SAE 50 oil is about 20 times thicker than water when in use and an SAE 20 about 10 times thicker so a much lighter viscosity grade.
The W refers to winter/cold temperature viscosity. It cannot be compared to water as it becomes ice at 32F/OC! However, again, the lower the number the less viscous it is at very low temperatures.
When an engine oil states both grades e.g. SAE 15W-50 or SAE 5W-30 it is called a multigrade.
The American Petroleum Institute (API) sets Service Categories for engine oils that are also used globally. Look for API S? something for your car in your service manual. If you have an older car, don’t worry, normally using a higher API is still OK. Latest is API SN but could also be OK for API SJ.
If you have purchased a European or Japanese car, you might see a reference to ACEA and/or ILSAC, but API & SAE should be there also.
Choosing the right API Service Category and then SAE Viscosity Grade is very, very important and depends on 2 things, your engine design and the ambient temperatures where you are driving.
The service manual in your car will guide you.
First, confirm the API minimum performance requirement.
Then it is like buying a suit or a dress –> the material, the design, the designer, the colour, the cost could all be the same, but the size which can be very different is also very important for it to work.
Same for engine oil re SAE Grade.
Read our ultimate guide to understanding engine oil viscosity here.