I did read somewhere, it was due to engines with PPF and DPF etc. I can't find the website now annoyingly..
This makes sense
Coming from a background in motoroil chemistry, I can confirm that oils can have the same functional properties but be chemically different from each other. With respect to the quoted post, oils made for diesel engines will more often than not, contain low SAPS compounds. That is sulphated ash, phosphorus and sulphur.
A low-SAPS motor oil is an oil with a low proportion of sulphate ash, phosphorus and sulphur. These oils are also designated “low-ash” due to their low tendency to ash formation. Particularly useful in modern cars with a DPF! Stops your DPF from clogging up so quickly and prolongs the life of catalytic converters.
Whilst this sounds like a great idea, High SAPS oils are easier to make, and thus tend to be used in petrol engines where the requirement for low ash is not as high. However, with the introduction of GPF. I imagine we will see more cars requiring Low-SAPS motor oils.
Based on the above, and the below flow chart, it would appear there is some chemical difference between 507 and 504 that would impact the longevity of the DPF. I would say it is safe to put 507 into a petrol car, but not the ther way around.
Below is more supporting information about the importance of SAPS between the two oils.
VW 504.00
The VW 504 00 specification superceedes the VW 503 00 and VW 503.01 specifications. VW 504 00 oils are suitable for engines meeting the demands of Euro 4 emissions standards.
VW 507.00
Low SAPS oils suitable for Euro 4 engines and almost all VAG diesel engines from 2000 onwards with extended service intervals, unitary injector pumps and also Pumpe-Düse ("PD") engines. Excludes V10, R5 engines and VW Commercial vehicles without fitted DPF (diesel particulate filters) – these must use a 506 01 specification oil.