When it comes to maintaining engines, whether in plant machinery, agricultural equipment, generators, or commercial vehicles, oil choice matters. A question our plant hire customers get from their operators and fitters is:
“Can I mix different grades of oil? Such as 10W-40 with 15W-40?”
The short answer is: Yes, you can mix them - but only in an emergency. Here’s why.
What Do 10W-40 and 15W-40 Mean?
Engine oil grades are defined by viscosity. The numbers represent how the oil flows in different temperatures:
- The first number (10W or 15W) = Cold temperature performance
- The second number (40) = High-temperature performance
Both 10W-40 and 15W-40 behave similarly when the engine is hot, but 10W-40 flows better in cold starts. Read more on this is our recent blog explaining SAE Grades.
What Happens If You Mix 10W-40 and 15W-40?
Most modern oils are compatible, and mixing won’t cause immediate damage. If you're low on oil, topping up with a different viscosity is better than running with low levels, which can lead to:
- Increased friction
- Overheating
- Rapid engine wear
However, mixed oils cannot guarantee:
- The same additive balance
- Optimal lubrication performance
- Maximum engine protection
So while mixing is safe short-term, it’s not a long-term solution.
When Is It Safe To Mix Oils?
You can mix oils if:
✔ You're topping up temporarily
✔ You replace both oils with a fresh change soon after
You should avoid mixing if:
✘ Your machine has very strict manufacturer requirements
✘ You’re operating in extreme temperatures
✘ You’re using synthetic with mineral oils without checking compatibility
What Should You Do After Mixing Oils?
If you’ve mixed oils of different viscosities:
- Schedule a full oil change as soon as practical
- Monitor oil levels and engine performance
- Use the recommended oil grade next time
Why Correct Oil Matters for Your Machinery
Using the recommended oil ensures smoother operation, reduced wear on moving parts, improved fuel efficiency and longer engine lifespan. That’s why HTS Spares supplies high quality lubricants from Kompo® - trusted across construction, plant hire, agriculture and logistics sectors. Whether you need 10W-40, 15W-40 or manufacturer-specific oils, HTS Spares has you covered with fast delivery, reliable products and expert support for your fleet. Explore our full range of engine oils and lubricants at HTS Spares.
Final Answer: Can You different grades of Oil? Such as Mix 10W-40 and 15W-40?
Yes - but only if absolutely necessary.
For best engine performance and protection, it is always best to stick to one grade and follow manufacturer guidance.
