techra:
will it be able to detect capacitance when the plates are seperated by 15cm or so
I think you need to be looking at fractions of a millimeter separation, not centimeters. And I still doubt that you will get a meaningful level reading out of it.
Is there some reason you can't use a commercial petrol level sensor? Surely lots of bikes have them, and you can be fairly sure that they work and are safe if used correctly.