Re 10v vs 12. The 12 v supply varies with battery charge so to get the gauges to read the same regardless of battery charge they run on a 10v supply. There's a liitle box behind the dash that has a heater coil and bimetallic strip (its VERY low tech) and it averages 10v. The gauges should come off of that, but I have a feeling the oil gauge does not but tempand fuel gauges do. Follow the wires. voltmeter wont work as its an average 10v and the voltmeter will jump all over the place from 0 to 12.5..