What is the difference between 5w30 and 5w40?
3 Answers
5w30 and 5w40 differ in viscosity: 5w30 oil has a kinematic viscosity between 10 and 12, while 5w40 oil has a kinematic viscosity between 13 and 16. The 'W' stands for winter, and the smaller the number before it, the better the oil's low-temperature fluidity, indicating a lower usable environmental temperature and better engine protection during cold starts. The number after the 'W' represents the oil's kinematic viscosity at 100 degrees Celsius. A higher number indicates higher viscosity, better protection at high temperatures, and can also be understood as representing the oil's stability at high temperatures (i.e., the likelihood of thinning). A larger number means better high-temperature stability performance.
I've driven several cars and always wondered what exactly the difference is between 5w30 and 5w40. Simply put, 5w30 has better fluidity during cold starts—the engine responds immediately when you turn the key in winter, no sluggishness. In summer, the oil is thinner, which can save some fuel, but under prolonged high RPMs, it may lack sufficient protection. On the other hand, 5w40 has the same 'W' value, so it performs well in low temperatures, but its high-temperature viscosity is thicker, making it suitable for hot weather or frequent highway driving. The oil film is stronger, reducing engine wear. For my daily city commuting, I use 5w30 for peace of mind, but after switching to an off-road vehicle for frequent trips to the countryside, I switched to 5w40 for better reliability. Just choose based on the season and driving habits during each maintenance—don’t use the wrong one, as it significantly affects engine lifespan and fuel consumption.
From a maintenance perspective, the core difference between these two oil specifications lies in their high-temperature performance. The number 30 indicates lower viscosity—the oil thins out as temperatures rise, which reduces friction and improves fuel efficiency, making it ideal for new vehicles or gentle driving. The 40 grade has higher viscosity, maintaining its thickness at high temperatures to provide an extra cushion for the engine, making it better suited for older vehicles or high-load driving conditions. I always remind people not to choose randomly—check the manual for the manufacturer's recommendation to be sure. For example, turbocharged engines that generate high horsepower should use 5W-40 to prevent overheating damage, while regular cars can generally manage with 5W-30. When changing oil, be aware that incorrect viscosity may lead to unstable oil pressure or premature degradation, so monitoring the dipstick level closely is crucial.
