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! ;-)