Yeah, you should have posted that in the modificatons main forum. I'm sure somebody will move it.
But in reference to your question, a simple way to make oil not a hard prereq is simply to make a second unit just like the first that has different requirements. Diesel Tank requires Oil and Steam Tank requires Coal. Also, you can make buildings produce resources. Building Sythetic Oil Refinery requires coal to build and produces oil (much as Broadway produces musicals). Unfortunately, you cannot do this with improvements using XML alone.
As for quantity of strategic resources why not create different resources with nqames just like that. For my hard sci fi mod I am making a line of Antimatter generators that produce different amounts of antimatter. NanoAMgen makes NanoAM, or nanoscopic quantities of Antimatter, which resource only does limited things. MicroAMgen makes MicroAM or microscopic quantities of Antimatter, which resources does more things than NanoAMgen.
OR prerequisites would be nice. We have them in tech, what about resources? Its not as simple as diesel and steam tank sometimes, and making a plethora of alternate units (so that a unit sufficing on NanoAM can also use MicroAM) is a clunky pain.
