What car are you talking about? My 84 & 85 300D's have a mechanical oil pressure gage on the dash. But, many cars use a low-pressure switch to a lamp and that also works. The journal bearings work fine as long as they have a good supply of oil, even at 5 psig. The oil pressure indicates how close you are to the flow limit, and indicates when bearing gaps have gotten too large.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans