Understanding the Difference Between Petrol and Diesel Engine Oil
Tom Vondrasek | 20th June 2025 | 3 minutes to read
Petrol and Diesel oils perform the same functions in that they lubricate and protect their respective engines against wear and tear. It is the difference in the fuel that causes the oils to differ in their make-up. Just like the engines that are made of basically the same components, they are just tweaked to get the most from the type of fuel they use.
The fuels used are the cause of the difference. Petrol is more volatile and a spark causes it to ignite with extreme violence. Diesel is more placid and needs to be put under immense pressure which causes heat before it reacts. The oils they use are essentially the same, though are modified to suit their respective engines.
Lets look into the differences:
Engine Types Comparison
Petrol
- High revving, high horsepower, lower torque engines
- Cleaner running, less emissions. Has a catalytic convertor to clean exhaust gas.
- High performance passenger vehicle engine
- Worse fuel economy than Diesel and shorter service life
Diesel
- Low revving, low horsepower, high torque engines
- Dirtier running, more emissions. Has a catalytic convertor to clean exhaust gas and DPF (Diesel Particulate Filter) to remove soot in that gas.
- Commercial truck engine, designed for pulling, now adapted to suit passenger vehicles
- Better fuel economy than Petrol and longer service life
Difference in Petrol & Diesel Oil Composition
Petrol Engine Oil:
- Generally a lower viscosity oil compared to Diesel engine oil. The oil contends with lower compression ratios, less stress on components and lower combustion temperatures.
- Less additives per volume of oil as the engine runs cleaner with a lot less combustion residue
- Less Detergents & Dispersants
- Less Anti-wear agents
Diesel Engine Oil:
- Generally a higher viscosity oil compared to Petrol engine oil. Needs to withstand higher compression ratios, stresses on components and higher combustion temperatures
- More additives per volume of oil as the engine runs dirtier with soot as a byproduct of the combustion process
- More Detergents & Dispersants
- More Anti-wear agents
API and ACEA Ratings (Specifications)
Both types of oils are categorised by international standards for their performance:
Petrol engine oils are labelled with:
- API (American Petroleum Institute) ratings starting with S, SN, SP
- ACEA (Association des Constructeurs Europ ens de l Automobile ) ratings starting with A/BThe Ais petrol, the B is light diesel so the oil can be used for both engine types. Eg A1/B1, A3/B4, A7/B7
Diesel engine oils are labelled with:
- API (American Petroleum Institute) ratings starting with C eg CK-4, CJ-4 also the new FA-4
- ACEA (Association des Constructeurs Europ ens de l Automobile ) ratings starting with C for TWC (Three Way Catalyst) & DPF (diesel Particulate Filter) engines. eg C1, C2, C5. It uses E for heavy duty diesel engines. eg E1, E4, E6
As you can see API ratings are simpler than ACEA when it comes to petrol & diesel oils. The important thing here is following your vehicle manufacturers recommendation. Look for the API or ASEA rating they state suits the engine in your vehicle. Use this to double check when purchasing engine oil for your particular make and model of car.
Interchangeability
On the question of can I use diesel oil in a petrol engine or vice-versa the answer is simple. As long as the oil viscosity and specifications match what is recommended by the company that made your car, then it is OK to use. If you are not sure, do not use it.
Diesel Oil or Petrol Oil
Petrol engine oil and diesel engine oil are formulated differently to meet the specific demands of their respective engines. It is easiest and best practice to buy a diesel oil for a diesel engine and petrol oil for a petrol engine. So when you see an oil on clearance or special, don t buy it because it is cheap. It still needs to suit your engine, otherwise the cheap oil purchase may end up costing you a whole lot more.
