Currency backed by gold, not an illusion. Currency backed by oil, not an illusion.
Is the value of gold not an illusion? Is the value of oil not an illusion?
Any value is an illusion. As long as that value can be applied long term, stored, and is limited in supply, it just makes the process of creating a price easier
These things have real value, in today's society, oil more so than gold. In fact our entire economic system is tied to oil prices. Unfortunately we are running out of oil, quickly, so perhaps generic energy options will work better.