I believe the one wire would just be a switch for a dummy light, which the car is already equipped with.
For racing you'll want both a gauge and a dummy light that is set around 15-20 psi. The factory light is set around 2psi, by the time it comes on damage has already been done. For a DD just a gauge is fine.
Electric gauges (with a sending unit) are far superior to the mechanical gauges that use a small plastic oil line. They are more expensive, but it'll ensure you never have an oil leak that ruins your interior.
The block passage is 1/8 BSPT. You'll most likely need to order a tee online to fit the new sending unit, since hardware stores don't carry them.
