Hello, your guess is as good as anyone else's.
Vandalism, stupid mechanic, stupid anyone, drunk anyone, over active imagination, etcetera, etcetera.

Go to technical section of the forum to post or search for gas tank draining.

 

How did you find out you had oil in there?

When did you last fill up with fuel?

Has anyone tampered with your fuel cap and broken the locking latch?

When was he car last in a workshop?

When you did last check your engine oil?

Was it up to the correct level?

Have you lent the car to anyone recently?

As for getting it out, any good workshop should be able to drain and flush through your fuel tank and the fuel lines.

Once the car is running ok, get them to check the exhaust emissions to make sure they are still okay.