What about a oil bonus?

[Carsten]

When i make 15 Stars in MP-battles, i get stuff.
When i make a exploration, i get stuff.
When i dig my gold mines, i get stuff.
When i harvest the trees, i get stuff.
When i watch ads, i get stuff.
Well, everywhere i get stuff now.

But... i never get any oil. Except the seeps which pops up around five times a month.
And digging for oil of course. But oil is still so rare and you need so many of it. It takes years to upgrade the industrial age.

 

[Brand Marrow]

Yup, oil is big challenge in game. But there are combinations of troops which can successfully raid even the best protected bases and there are lots of bases with 3 or 4K oil ripe for the siphoning!