I'm not sure about the 6 to 12 volt conversion but I do know if you use a volt ohm meter, you can test the sending unit. To test: put the meter into the ohm scale, put one lead on the post of the sending unit and the other lead on a ground. The reading should be very close to zero. With the leads still in place start the engine and note the reading, it should not be zero but maybe in the 20-40 ohm range. I'm going by memory on the numbers. As you increase the engine rpm's the ohm reading will change as the oil pressure increases. To test the gauge, ground the sending wire and see if the gauge moves.
