ELECTRICAL only... with a mechanical there is no sender, well not like on an electrical.
say you install a mechanical oil gauge... you have to run a little tuve from the oil sender unit all the way to the gauge, so..... you will have hot oil running all the way from the gauge to the gauge pod to the gauge, if this line becomes messed up or gets a rip then you will spew hot oil all over the inside of your car
the ONLY form of this ive seen thats ok is a boost gauge, its just air in the lines.
ONLY go electrical. I have eletrical oil and water temp gauges they both work great, both are autometer also lunar series.