petrol in oil?

[unfazed]

10w/40 is 10w/40 

Not true :rolleyes
The are 3 basic types of 10/40 oil
Mineral 10/40
Semi Synthetic 10/40
Synthetic 10/40
Neither should be mixed with each other, whereas you may top up semi synthetic with fully synthetic and fully synthetic with semi synthetic, under no circumstances should mineral oil be mixed with fully synthetic of semi synthetic.
As I said the specification should match before you decide to mix