I'm getting mixed numbers and words here.
Tony, you ask for a guage? for in the dash? there is "guage, oil" listed in microcat as pt # PRC7319 (under body,electrics,instruments,instruments clocks) which is oil pressure, available from various UK outlets
PRC4043 is the oil pressure sender for 2.5 NA and TurboD's, not 300's... here's one
300Tdi oil pressure switch (for the red light... either on or off, not a variable guage) is PRC6387