Oil?

[Ancient One]

What's the difference between oil and the other resource (shown by shovel and pick)?

[Ancient One]

Well?

[Hubert Cater]

Each different resource type from ports to oil will be represented by a different value. The total of these values will make up your income per turn. Oil will be the most valueable and will yeild the highest return in this regard when controlled, so for example getting countries like Romania on your side or gaining control of the rich Caucases (mines and oil) will feed your coffers while denying those of your enemies ;)

[Ancient One]

What are the different values? Is it like 10 for each city, 20 for each mine, 30 for each oil?

[Hubert Cater]

Roughly, it will also depend on overall strength/effeciency as well as damages from bombing etc.