I agree with droopsnoot (Mike) The pressure switches are cheap and worth changing just in case. The oil itself could well be the problem. The slant 4 puts out a lot of heat and modern formulae of 20/50 will not hold up very long to the extremes of temperature.
Modern mineral oils have a much lower shear resistance than their ancestors, mainly due to the need to reduce emissions of the deadly additives. Once the shear temp is reached the oil never recover it's original viscosity, and the shear temp is lowered, this is than a downward slope eventually to oil pressure problems when the oil is hot.
You really need a semi synthetic with ZDDP additive these have a much higher shear resistance than mineral oils. You can than be sure that the oil does not reach it's shear temperature.
I had the same problem with my 2300, a modern high quality classic blend 20/50 mineral oil would fail after only a few hundred miles. I did a lot of research and asked lots of questions from oil manufacturers and blenders. From their recommendations I decided to use Millers Classic Sport 20/50 Semi Synthetic and have not had any problems since.
Have a read at this post.
viewtopic.php?f=289&t=26312&hilit=shear
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"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.