Using the correct engine oil is imperative to a trouble free, long life from your engine. Sometimes you may need to top off your oil, choosing the correct weight and type specific to your engine ensures no issues further down the line.

But what happens if you don’t have the exact weight of oil needed for your engine. Can you mix different weights of oil?

Weights Explained

Mixing oils isn’t a straightforward yes or no answer, as there are many variables. The first is the weight of oil, identified by “0w30” for example. The first number represents the viscosity in winter, the lower the number indicates thinner oil when cold.

The second number indicates its viscosity at 100C. This is a standard measurement across all manufacturers. The higher the number indicates how thick the oil is, therefore 0w40 is thicker than 0w30 at 100C.

When you mix two different oil weights, its viscosity changes depending on the mixture. For example, if you have a 50:50 mix of 0w40 and 0w30, you’d expect the mixture to end up around 0w35. The issue lies if there is a large discrepancy between the mixed oil weights, altering the viscosity outside of manufacturers specification.

For a quick top off there shouldn’t be any issues if the viscosities are close like our example – providing the manufacturer is the same. Be aware using an extremely thick or thin oil can cause irreparable damage. Using your best judgement here is key.


Oil Types Explained

There are two types of engine oil used, Mineral and Synthetic. Mineral oil is used on older vehicles as it was the standard until more performance was required from combustion engines. A solution was needed to the increased temperatures and strain on oil, so a purer and better performing solution was required.

Synthetic oil has 5 main benefits:

  • It’s resistance to breakdown under higher temperatures is better, ensuring proper lubrication in hot climates or performance cars.
  • Performance in low temperatures (-40C) ensures protection despite being below freezing.
  • Better protection from degradation. Synthetic oils resist shearing much better, maintaining it’s viscosity over the life of the oil.
  • Lower consumption of oil as synthetic blends experience less boil off compared to traditional blends. According to Total, a quality synthetic oil will only lose 4% of it’s weight when exposed to 400 degrees for six hours. Mineral Oil will experience 30% loss under the same conditions.
  • Cleaner engines as synthetics don’t sludge up as fast. This means short trips or driving in winter results in better lubrication and protection.

Is it okay to Mix Different Brands of Motor Oil?

You can mix the same type (synthetic or mineral) from the same manufacturer. Different manufacturers use different additives to achieve the best performance from your engine oil. The issue lies in how these interact with each other inside the harsh environment of an engine – potentially causing failure.

Be careful you’re not mixing different types of oil, only ever mix synthetic with synthetic, and mineral with mineral. If in doubt don’t risk it, you can get oil the next working day from X.