In order to create taxes, which depend on other taxes (tax on tax), you create tax groups first.

This is how it is done on Odoo 11.0 CE:

1. Create a tax group for application of fuel tax and VAT:
Go to "Invoicing/Configuration/Taxes". Add a tax group, name it "VAT on fuel" for example, create the tax scope, in your example "Purchases". Under "Tax Computation" select "Group of Taxes". Save the tax group.

2. Add the first tax to be applied (Add Children Taxes):
Edit the tax group again. Under "Add an item" add the first tax to be applied (create a new one, if necessary). Name it "Fuel Tax" for example, set the tax scope ("Purchases" in your example). Set the required tax computation method and amount.

Under the tab "Advanced Options" define the tax label on invoices ("Fuel 20%" for example), select an existing regular tax group and check "Affect Base of Subsequent Taxes".

3. Create the second child tax the same way, name it "VAT" for example:

Under the tab "Advanced Options" define the tax label ("VAT 10%" for example), select an existing regular tax group and do NOT check "Affect Base of Subsequent Taxes". Save & Close.

4. Check your tax group with the children taxes and save it.

5. Assign tax group "VAT on fuel" to a product:

6. Create a purchase order with the same product and check the correct taxes applied:

As you can easily see, first the Fuel Tax of 20% was applied to the purchase price of 1000, which results in a gross amount of 1200, and after that the VAT of 10% was applied to the gross amount, which results in 1200 + 120 = 1320.