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Most diesel engine oils also have an API rating for petrol engine use (the 'C' rating for compression ignition and the 'S' rating for spark ignition).
Presumably, as long as the API petrol service rating and the SAE viscosity rating meet the specs for the engine, a diesel oil can be used quite satisfactorily in a petrol motor.
After all, most oil companies produce lubricants that are marketed for "mixed fleet" use for transport or contracting firms that run both diesel and petrol engine vehicles.
It has been suggested to me that because diesel engines place greater demands on their lubricants than petrol engines it is actually beneficial to use a diesel oil in a petrol engine (again, provided manufacturer's specs are observed).
In the US it is very common for a particular brand of diesel oil to actually be run in motorcycles, particularly touring bikes that cover huge mileages. This particular oil doesn't interfere with the wet clutches found in most bikes and is apparently achieving unprecedented benefits in low consumption and engine cleanliness.
Other international sources suggest that 4x4 enthusiasts are finding a particular brand of diesel oil to be an excellent option in both diesel and petrol engine vehicles.
So, can a diesel engine oil offer advantages in a petrol application?
Gary
From the "Ask Jack" archives - 22 December 2009
You raise a very interesting question. In theory, provided a diesel engine oil has the API (American Petroleum Institute) rating for petrol engines (S), it can be used.
However, it is not a recommended practice in general within the industry.
Modern lubricants are manufactured for a specific use with unique additives designed to meet a certain engine criteria which can be hugely different from petrol to diesel. Diesel engines for example require a much higher detergent level which will do petrol engine no favours as far as lubrication is concerned.
Not all diesel oil is the same either with modern common rail engines requiring their own specific oil in comparison to the older diesel technology.
The correct oil for the right engine type is critical to an engines overall life. A high detergent oil may be OK short term in a petrol engine, but will have fewer overall benefits long term.
Sticking to the manufactures recommend fluids for engines, transmissions and cooling systems is the best mechanic you will ever have.
There are always people who deviate from recommended practices for a specific individual need and it works fine for them. Following down the same pathway may not provide you with the same benefits, in fact the opposite could easily apply.