5W-30 VS 10W-30
5W–30 and 10W–30 lubricants have the same viscosity at the operating temperature of the engine. That is usually 100°C (212°F) when the engine is running (in an average engine). However, the 10W–30 oil will be thicker when it is cold. As per API recommendations (1) for car engines, you can use 10W–30 oil if the maximum expected outside temperature is above -18°C (0°F). If you are living in an area where the outside temperature can go below zero; It is better to use 5W–30. Always follow the engine manufacturer’s recommendations.
5W–30 vs 10W–30 at 100°C:
Shown in the video are 5W–30 and 10W–30 engine oils in tubes at 100°C (212°F). A steel ball moving from top to bottom indicates the thickness or viscosity of the oil. At 100°C (212°F), the viscosity or resistance to flow is almost the same for both 5W–30 and 10W–30.
The number 30 in both 5W-30 and 10W-30 indicates the SAE viscosity at 100°C (212°F). That is the operating temperature of an average engine. The number before “W” (5W-XX and 10W-XX) tells the SAE viscosity of the oil when it is cold. W indicates winter or cold and that’s why it is shown here in blue color.
In the example above, 5W-30 has “SAE 5” viscosity and 10W-30 has “SAE 10” viscosity (5W and 10W) when cool. A higher SAE number means that the oil is thicker or has a high viscosity. SAE 10 is thicker than SAE 5, and similarly, SAE 40 is thicker than or has a higher viscosity than SAE 30. Check the SAE Viscosity chart called SAE J300 standard.
So both the oils 5W–30 and 10W–30 have exactly the same viscosity (SAE 30) when your engine is running, and will perform in the same way. However, the 10W–30 oil will be thicker (higher viscosity) than the 5W–30 when it is cool.
“W” relates to winter or cold
Shown in the video are tubes with 5W-30 and 10W-30 lubricants. Both tubes with the steel ball were refrigerated at -18°C for half an hour. Air bubbles are trapped in the test tubes because of the low temperature. The thickness of 10W-30 is much higher, as shown in the video demonstration.
Which is better?
In modern car engines, we need the oil to be thinner at the time of startup. Most experts agree that 80% of engine wear occurs during startup (warming-up) the engine. There is a problem with the thicker oil when the engine is cold. We need the oil to be thinner at the time of starting the engine. The lubricant must reach all parts of the engine right away. However, the problem with thicker oil is that the oil’s resistance to flow is high. Hence, the oil will not reach all around the moving parts of the engine when you turn the key. 5W–30 oil has the same viscosity at 100°C but is thinner when cool. 5W–30 is always a synthetic lubricant. The only problem with 5W–30 is NOACK (evaporation tendencies of a lubricant (engine oil) at high temperature).
According to API motor oil guide , If you are using 10W–30, you can safely switch to 5W–30 (in car engines). 5W–30 is thinner than 10W–30 when cold. (you must follow the manufacturer’s recommendations). However, switching from 5W–30 to economical 10W–30 is not always recommended. According to API motor oil guide , you can use the 10w–30 if the lowest expected outdoor temperature is above -18°C and you are following the manufacturer’s recommendations. I am using 5W–40 for my car instead of 15W–40. It is a recommended lubricant along with 15W–40 by the engine manufacturer, even though I am driving in a hot climate in the gulf region. 5W is a synthetic lubricant having better oxidation resistance and longer drain intervals.
Can I use 10W-30 instead of 5W-30?
The only reason to choose 10W-30 is the price. Otherwise, 5W-30 is superior to the 10W-30 for car engines. Better select synthetic 10W-30. You can only switch to mineral 10W-30 if the lowest expected outside temperature is above -18°C (0°F). Always follow engine manufacturer recommendations.
Can you use 5W-30 instead of 10W-30?
Absolutely yes for car engines, 5W-30 is usually synthetic and superior to 10W-30. Both have the same viscosity at the operating temperature of an average car engine. 5W has lower viscosity when cool and is considered better for the car engine during engine start. For high-power diesel engines, follow manufacturer recommendations because of higher NOACK in 5W.
Is 10W-30 thicker than 5W-30?
Both 10W-30 and 5W30 have the same viscosity at 100°C (212°F), it is the operating temperature of an average engine. But 10W oil is thicker or has higher viscosity when cool.
What does the “W” stand for in the SAE viscosity grades?
“W” stands for winter, for example in 5W–30, the lubricant has SAE 5 viscosity when cold and SAE 30 viscosity when the engine is running. There are 2 sets of numbers in 5W–30 separated by a dash(-). The number 30  relates to the viscosity or thickness of the oil when it is at 100°C. It is considered a working temperature for the average engine. The “W” means winter and 5W relates to how the oil performs when it is cool. So 5W–30 is SAE 30 viscosity when at 100°C and SAE 5 viscosity when cool.
(1) Motor Oil Matters by api.org
(2) SAE Viscosity Grades.