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All vehicle manufacturers today recommend the use of 'multi-grade' motor oils. To find out which SAE multi-grade oil is recommended for your particular vehicle, you should consult your Owner's Manual. The recommended oil grade will usually also be printed on the Oil Filler Cap or on a sticker under the hood. Now the manual might list only one grade of oil for use in all temperature ranges, or it might list a few different grades of oil to choose from based on ambient temperatures (note that this recommendation might be different for different countries).

As mentioned in my previous article, "Motor Oil Viscosity Grades", the viscosity grade numbers (i.e. the '5W' and the '30' in a SAE 5W-30 oil) are simply a rating representing the "viscosity range" of the oil. They are NOT the actual viscosity of the oil. The viscosity of an oil is temperature dependent - a particular grade of oil will have a different viscosity (thickness) at different temperatures.

For a SAE 5W-30 multi-grade oil, the first number (5W) is the 'cold' viscosity rating of the oil, the 'W' stands for winter, and the last number (30) is the 'hot' viscosity rating of the oil. Therefore, when the oil is cold (e.g. in sub-zero temperatures) it has a rating of '5W'. When the oil is hot (e.g. circulating in a hot engine) it has a rating of '30'.

What this means is that a 0W-30, a 5W-30, and a 10W-30 multi-grade oil will all have essentially the same viscosity (thickness) when circulating in a hot engine (about 10 cSt at 100°C / 212°F) but will have a completely different viscosity when cold. It is the second number (30) that will determine the ‘thickness’ of the oil at operating temperatures.

In other words, all three grades will offer the exact same protection when your engine is at "operating temperature", but the grades with a lower 'xW' number will flow better when the engine is cold. Because a 0W-xx multi-grade remains more 'fluid' in colder temperatures than a 5W-xx, having a lower 'Cold Cranking Viscosity' and 'Cold Pumping Viscosity', the oil will get up into the engine more quickly upon start-up, therefore minimizing engine start-up wear. It will also reach the proper 'hot' viscosity faster, further minimizing wear.

Looking at it another way, a 5W-30 multi-grade and a 0W-30 multi-grade will both have the same 'thin' viscosity when circulating in a hot engine, but as the engine cools down, the 5W rated oil will 'thicken' a lot more, and a lot faster, than the 0W rated oil.

It's also important to understand that at "ambient temperatures" (even at HIGH ambient temps), ALL grades of oil are too thick/viscous (yes, even a 0W grade is too thick). In other words, a 0W grade is thicker at ambient temps than a 30 grade is at operating temps. It is only after the engine warms up, heating up the oil, that the oil "thins out" and reaches its proper 'hot' viscosity.

It is said that over 75% of engine wear occurs at start-up when the engine is cold (and in the 10-20 minutes after start-up while the engine is warming-up). Extended idling (e.g. warming up your vehicle in the winter) will also accelerate wear. Therefore, if you are interested in engine longevity, you should focus on reducing and minimizing this start-up wear. And the best way to accomplish this is by choosing a multi-grade oil with a lower 'cold' viscosity number that will flow better in cold temperatures, and be easier to pump, and be able to get up more quickly into your engine.

Note: I strongly recommend the use of an Engine Block Heater in sub-zero temps. The faster the engine and oil get to operating temperature, the better.

Although the above chart doesn't show a SAE 0W-20 or 5W-20 grade, if you extrapolate, the temperature range of a 0W-20 multi-grade would be "-40°C to +30°C" and a 5W-20 would be "-35°C to +30°C".

The bottom line is: the question as to which oil viscosity grade can or should be used in your vehicle’s engine will depend, among other things, on the type of climate you will be driving in - if you live in a very hot climate you could, if you wanted to, use a higher viscosity grade oil (10W-40 for instance), and if you live in a much colder climate you could choose a lower viscosity grade oil (0W-20 for instance). However, the recommended viscosity grades should do just fine in all climates.