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Old Hoakie knows his stuff. Check that relief valve. If the relief valve checks out ok, try a different gauge. But just in case everything checks out ok (and I bet it all will), read on. These tractors are all more than fifty years old. On an older vehicle, oil pressure that goes ABSOLUTELY to zero is one of the things that will make me start to think about a rebuild. Another is bad compression numbers. If the oil relief valve and pressure guage is ok, and no one else has a better suggestion, do a compression check. I don't think low compression will CAUSE low oil pressure, but low oil pressure and low compression TOGETHER often indicate a rebuild is necessary where one factor by itself might not. However, if the oil pressure is dropping to say, ten pounds or so at idle after warmup, and not completely to zero; and rebounds immdediately as soon as the revs increase, I wouldn't sweat it too much. Pretty much all older engines to that. Above all, DO NOT operate the tractor for any length of time with zero PSI. Especially under load. Unless it is a trailor queen, a tractor that has NO oil pressure is almost useless. You can't use if for any work to speak of, unless it's moving equipment around in the yard, pull-starting another tractor, or other tasks that can be completed before the engine reaches operating temperature. And, a list of these tasks is pretty short around most operations. My sister, a surgeon who is also an excellent diagnostician, always says, "When you see (manure), look for a horse, not a zebra." In this case, a simple fix is the zebra. I'm afraid you're looking right at a horse.
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