Underground Oil-Water Drain Tank

Dipstick.

Submersible head pressure hydrostatic transmitter and your choice of indicator/controller/recorder

http://www.lesman.com/unleashd/catalog/level/level_WIKA_LS-10_LH-10_Submersible.html

Don't use capacitance, Boss.

"level gauge and a level transmitter shall be considered separately"

Then, a dipstick will not work, because it is a combined unit.

I suggest a lighted match lowered into the top hatch. No, wait, that won't fit the requirements either.

The question is.....................................................................a sure indication that you are in the wrong line of work.

You need a level displacer-type instrument. You have not stated any budget contraint, so I refer you only here.... http://www.iamechatronics.com/notes/general-engineering/323-displacement, for more info on level displacers, and here for actual physical instrumentation en.wikipedia.org/wiki/Emerson_Process_Management

Naturally, there are many other manufacturers/ suppliers, and I for one, believe you can probably attach a displacer to a dipstick....but then I am not a manufacturer or supplier, have no commercial interest, no R&D dept and no underground tank!

GWR is a good choice. If the tank is enclosed in a concrete pit with access ladder and sufficient clearance, can go with a GWR+MLI or GWR and separate level gauge. Redundant LTs may be required if fire or explosion risk is high.