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Hi, this concerns the oil pressure on a 1939 Super-8 motor. I have the correct amount of 10w-40 Rotella oil in the crankcase. Now, reading the dash pressure indicator when I start the engine from cold I get about 20-25 lbs at idle. On a 15 mile trip with the engine warmed up it reads 40-45 (at 50 mph) but at the 14 mile point I noticed it indicated around 30-35 lbs (still 50 mph). First thought was oil pump wear (engine has 200 miles since rebuild but not known if oil pump was reconditioned or just stuck back on). An incredibly knowledgeable person from the forums (not sure I should mention his name, I should ask!) suggested the problem could be the oil pressure regulator and that it should be removed, cleaned and adjust the spring inside. My mechanic that I've used for 20+ years suggested if I don't want such a disparity in pressure just go with straight 30w oil.....he's a bit old fashioned when it comes to newfangled multi-viscosity oils. He also said 30 lbs at 50 mph will not hurt the engine, yet the Packard manual suggests 55 lbs ay top speed. I myself prefer multi-viscosity oils, but that's just me.
Which way should I go?

Thanks,

Steve

This may not apply here, but I had a '47 Bentley that tooled along at 60 with 5 lbs pressure

When that car was built they didn't make multigrade oils. The regulator would dump excess pressure when cold and as the oil warmed and thinned less pressure would be bypassed. So the Packard specification for oil pressure is for a fully warm engine running on the road with oil temperature up near 160F. The rule of thumb for minimum oil pressure is 10 psi per 1000 rpm so for a Packard running 50 mph 25 psi is the absolute minimum, but that number two rod is the one that likes to go into orbit. Brand new modern cars do lose idle oil pressure as the enigines get hot but hold pressure on the road.

The rear cam bearing is a culprit as is the oil pump. They all need to be reconditioned but costs and sources are a problem. EGGE Machine was doing the eight cylinder pumps.

When they were new Packards had great oil pressure which is why the old timers were always trying crank it up. But if the pump is worn it can't keep up.

Oh, and if anyone doubts the oil pressure on those cars when new I have a picture from 1930 around here somewhere. I think it shows MPH zero, no charging - so idling, and near 40 psi, with the temperature near the N in range.

When your engine is started cold it should have about 55lbs pressure at a fast idle. when the engine is hot it should have 15-20lbs at low idle, hot and at speed the engine should have 35 to 45 lbs of pressure. I would adjust the oil pressure regulator to increase the pressure at idle. To increase the oil pressure remove the acorn nut from the regulator, loosen the retaining nut on the pressure adjuster and tighten it, this will cause the oil pressure to increase, This is a trial and error adjustment. There are many very knowledgable and helpful people on this forum. This information is what I garnered from this forum and people not on the forum, I took all the information and arrived at the above conclusion, its working fine for my engine.This is just my best opinion.
Andy

Just for comparative purposes:

Though there are some differences, my 1934 Eight engine is essentially the same as the '39 Super. I don't use the car in the winter so I use straight SAE 30 detergent oil. I have set the pressure relief valve to open at 55 psi which I always reach even at idle with a cold engine. Oil pressure at road speeds of 40 mph or so is usually 45-50 psi, dropping down a bit at higher speeds or extremely hot weather. Hot idle pressure is usually about 25-30 psi.

EDIT: From the 1934 Eight owner's manual: The gauge will usually show in the neighborhood of 25 to 55 lbs. pressure at normal driving speeds with a pressure of 2 lbs. or more when the motor is idling and the oil is warm.

Thank you gentlemen!