1 Energy = 4.2 Oil, 3.6 gold, and 6 supplies when digging as averaged over 10 typical dig results. Because of this, it is not recommended to trade for oil, as you can get more than double the oil (25.2) by digging with one hit of +6 energy from the Energy Links page than you can get with the maximum hit of oil, +12. Likewise, for 6 energy you will receive around 36 supplies when digging. Livestock can give an energy:supply ratio of 1:80 (buffalos).
See also: Commodity Metagame.