Oil removal in the map editor seems to be broken for map generated oil deposits (or at least it doesn't work like all the other resources).
The brush/spray settings don't work at all for removing generated oil, so it takes a ridiculous amount of time to delete oil in the map editor. You need to use the cursor, change to 800 intensity (max), and right click and hold on every single individual oil resource to remove.
On a rich resource generated map, it takes over TWO MINUTES to delete ONE oil resource (1500-2000% yield) with the cursor.

It's strange, because the brushes DO work to add AND remove new oil - but not for generated oil.
For all the other generated ore resources, the removal brushes work as expected.