What is the Difference Between 30 and 40 Oil Viscosity?
3 Answers
The differences between 30 and 40 oil viscosity are as follows: 1. Viscosity: 40-grade oil has higher viscosity than 30-grade oil. 2. Applicable temperature: 40-grade oil is suitable for higher maximum temperatures compared to 30-grade oil. 3. Fluidity: 30-grade oil has better fluidity than 40-grade oil, which helps reduce fuel consumption and dissipate engine heat faster. 4. Stability: 40-grade oil offers better oil stability under extremely high engine temperatures compared to 30-grade oil. Here is more relevant information: 1. Oil viscosity rating: The numbers indicate viscosity specifications. For example, in "15W-40," the "W" stands for Winter. The smaller the number before "W," the better the oil's low-temperature fluidity, meaning it can be used in lower environmental temperatures and provides better engine protection during cold starts. 2. Letter representation: Oil grades are represented by a combination of two letters. The "S" series represents oil for gasoline engines, with specifications ranging from SA to SN (in alphabetical order, excluding SI). Each subsequent letter indicates improved oil performance, with more additives to protect the engine.
With years of experience in automotive maintenance, I've learned that the difference between SAE 30 and 40 oil viscosity primarily lies in high-temperature performance. The number indicates the SAE grade – higher values mean thicker oil. At 100°C, 40-grade oil is approximately 10% more viscous than 30-grade, with poorer flow characteristics but capable of forming a thicker protective film to reduce engine wear during high-speed operation or heavy loads. 30-grade oil flows better, allowing easier engine operation and improving fuel efficiency by about 2-3%. Always consult your vehicle manual: Japanese models often specify 30-grade for newer cars, while German vehicles or older models may recommend 40-grade to prevent oil leaks or clearance issues. I've seen cases where incorrect viscosity caused excessive noise or abnormal fuel consumption – regular oil level/quality checks with seasonal adjustments are advisable. For urban family driving, 30-grade offers fuel-efficient practicality; for frequent hill climbs during long-distance trips, 40-grade provides better protection.
I've been driving for over ten years and have changed engine oil with different viscosities countless times. Using 30-weight oil makes the engine feel lighter and acceleration smoother, especially when starting at traffic lights, saving fuel and providing a comfortable ride. However, during long summer trips when oil temperatures rise, the noise becomes as loud as a buzzing bee. On the other hand, 40-weight oil, being thicker, reduces noise and offers stable protection, but it makes cold starts in winter more difficult, requiring longer warm-up times, which can sometimes affect short-distance efficiency. I've found that using 30-weight oil in winter and 40-weight in summer works best in northern regions. For example, during a trip last year in high-altitude, high-temperature conditions, switching to 40-weight oil stabilized the oil pressure significantly. Nowadays, multi-grade oils like 5W-30 or 10W-40 can automatically adjust, reducing the hassle of switching. Driving style also affects the experience: aggressive drivers may prefer 40-weight for peace of mind, while those who drive gently may find 30-weight more economical, with noticeable differences.
