AA Members often ask us questions like: “Can I use diesel oil in my petrol engine?” or “What type of oil should I use?”

Aside from the obvious petrol vs diesel engine oils and multi-grade uses, there a two main oil qualities based on what they are made up of: Mineral and Synthetic.

We take a look at what do the multi-grade numbers and letters on the oil pack mean.

Viscosity index

The viscosity index of oil is rated by its resistance to flow. So, let’s use 10w-30 as a popular example. This has two viscosity grades - 10w and 30 - which gives the best of both summer and winter viscosity characteristics, and eliminates the need for both ’winter’ and ’summer’ oils as used in past. ‘W’ stands for winter, and the lower the number beside it represents the better the oil’s cold start performance.

The number following it must be the higher temperature viscosity, taken at a temperature of 100 degrees. The viscosity limit is set and all oils (regardless of brand) with a viscosity number must achieve these limits. Once again, the lower the number, the thinner the oil - a 30 weight oil is thinner than a 40 weight oil at 100 degrees, and so on. This is quite important, as engine oils naturally thicken as they cool and thin as they are heated.

Thin, low viscosity oils flow easier to protect engine internal parts at cold temperature. Thick, high viscosity oils are typically better at maintaining film strength to protect engines at high temperatures.

Mineral vs synthetic oils

Engine oils have different qualities in their makeup, and you can get a few grades, just like maple syrup. You can tell the genuine Canadian syrup - it flows nicely over your crepe stack, and the smell and long-lasting taste is amazing! You pay more for this luxury, but it’s worth every dollar. On the flipside, you can get a less expensive imitation or maple ‘flavoured’ syrup, which is not quite the same but still has its uses - maybe a child’s party desert, but certainly not a formal dinner party.

All oils are made from crude that comes from the ground. The difference is in the refining and additives.

Synthetic oils are made from a more advanced refining process, which removes more impurities from the oil and also enables individual molecules in the oil to be tailored to the demands of the modern engine. This purer oil is generally of a higher quality, and thus offers higher levels of protection due to increased anti-friction properties. The bottom line is that it can translate into less engine wear, increased fuel economy and longer engine life.

Mineral oils have their place, maybe once a vehicle gets older and the mileage is higher, or if the engine is a bit tired and burns too much oil to warrant using anything expensive.

Half and half

Semi synthetic or part synthetic oils are a blend of both mineral and synthetic oils. They provide better performance, protection and fuel economy than mineral oils, but are not as good as a full synthetic. This is a good in-between oil for those well kept low mileage cars that don’t quite require full synthetic engine oil.

Diesel oils

For a number of reasons it’s best not to use diesel oil in a petrol engine as even if the viscosity rating is the same, the oil properties may not be suitable.

The demands placed on a ‘compression ignition’ diesel engine mean that a slightly different type of oil is required than that of a ‘spark ignition’ petrol engine, because diesel engines create a great deal more soot and combustion by-products. Their oils have additives like detergents, which neutralise acids and keep surfaces free of deposits in order to keep the engine clean. When put in a petrol engine, the detergent will work as it is designed and try to clean the cylinder walls. Over time, this can have an adverse effect on the engine internals, resulting in lost compression and efficiency.

Oil viscosity is also a major factor here, engine oils need to be designed so the oil will easily pump around the engine at the lowest start-up temperatures while still protecting the components at in-service operating temperatures. Traditionally, diesel oils can have a higher viscosity and could play havoc in the cold starts as the oil becomes thick and can lead to premature wear.  

Diesel oils also contain high anti-wear additives in the form of zinc dialkyldithiophosphate (ZDDP). As a small amount of oil is burned in the process, diesel catalytic converters are designed to cope with this combustion by-product and petrol systems are not.    

It’s best practice to find out what oil the vehicle manufacturer recommended when the car was built and use this, as the properties found in that oil were the best choice for that engine. While brands and prices vary, there should always be an option to give you the best results.

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