Because you have only a limited number of moves, the importance of a single node is also a function of the importance of its neighbours, so that a nearby (to the player) but lonely oil node isn't as important as a more distant oil node that is itself near to other oil nodes.

This is starting to sound like some statistical clustering is in order, then you measure distances to the various clusters along with the maximum number of moves required to suck up all the oil in each cluster. You then increase the cluster sizes and choose the largest cluster you can reach.

Maybe I've just restated another posting, but I feel better! ;-)