Voltmetre is simple negative and positive, depending on whether you want it to read all the time or just when ignition on wire accordingly, may also have a backlight to wire in, same again neg to earth, live(for bulb) to sidelight circuit light, from ciggy lighter etc.
The oil pressure gauge again will have a backlight so wire this as the voltmetre is, and then on the adaptor you need a pressure sender, to give the variable earth to the gauge, the other gauge wire live will be connected to an ignition controlled source.
tabetha