This is the oil switch on Europe market 1985 BJ73 (although the image does not position it on the right side engine)
This is the sensor (they called it «switch» too and still not position it on the right side of the engine)
A coupe of hints from myself :
Oil :
Here the oil grade recommandation from the 3B FSM :
15W40 is «all good» down to -12C. I personnaly used (but shouldn't) use that grade down to -25C without having seen the engine cut off (still needed to rev a bit on start)..
When the 3B was created...
One of the first mod that I did on my diesel engines is adding mechanical gauges for oil pressur and collant temperature. Much more accurate and real time info.