Fluctuating oil pressure in a 10th Series Packard

[1935Packard]

A friend who has a 1933 Standard Eight (10th Series) has had a problem with his oil pressure, and I wondered if anyone here had any thoughts on the issue. My friend's description of the problem is below:

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The engine starts and runs fine, maintaining oil pressure continuously at 55 to 60-lbs, but last fall, after driving out on the interstate at 45-50-mph, the oil pressure dropped alarmingly to 15 to 20-lbs. Engine temperature is always normal and the radiator louvers function properly. New sylphon thermostat installed at time of restoration. So I dropped the pan and checked everything but found nothing amiss. The oil pump gear looks like new and spins freely and the lock nut on the pressure adjustment spring was tight. Removed the oil gallery line, blew it out but found no obstructions or sludge. Did same with pressure line going from engine through the firewall to the pressure guage. Everything clean. The babbited mains and bearing caps with Chrysler slant six inserts all tight and torqued properly. No obstructions. Inside of pan clean with no metal shavings but wiped it out to a mirror shine anyway before reinstalling it. Last few oil changes I've used straight Pennzoil 30-weight. I see no reason to go to 40-weight since the engine is still tight. So I'm still not sure if the fluctuation in oil pressure is normal or if it's something to concerned about. When I drive the car around through the park for an hour or so the oil pressure stays right up at 55-60-lbs.

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Any ideas?

[34PackardRoadsta]

I would suggesting having your friend install an independent oil gauge. The fault may be with the gauge.