International 140 Oil Presure

Oil pressure in my International 140 was too low.
So I rebuilt the oil pump which then delivered 45psi at full throttle, and 35 at low engine speeds. After about 30 minutes of run time the low idle pressure drops to about 15psi, while the full throttle pressure holds at 35psi. I am using a multi grade 10W30 oil. Maybe I should change oil to a straight 30 weight.
Any idea of what may be happening?

You are in good shape thats plenty of oil pressure for that engine.

Okay. The PSI at low idle just worried me.
Thank you.

Next time you change oil, try Rotella 15-40 (or equivalent 15-40). Seems to be popular - works for me. HTH

The crank & bearings are getting worn. Just keep an eye on it.

Will Do. But please help educate me a little. How does worn bearings alter the oil pressure after 30 minutes of run time.

Thirty minutes is about long enough for the oil to get warm and thin out. The pump cannot produce as much pressure AND the thinner oil leaves the bearings faster and lowers the oil pressure.

Also you say you "rebuilt" the oil pump. Not much you can do here EXCEPT sand the probably warped pot metal cover flat. If you didn't do that, then tear into it again and fix the cover. When I do one, I always make a flat metal strap and place it across the cover where the high pressure port is and hold it on with two of the cover bolts.

The oil pressure drops at idle as the temperature of the oil goes up. The warm oil is thinner and flows much easier.

The pump is a positive displacement pump. It delivers the same volume of oil per revolution at all speeds. When the engine is at idle, the volume drops. When the engine is at higher RPM, the pump makes more volume than can be used by the engine, so a portion is bypassed back to the pan through the relief valve. That's why the pressure levels off and holds steady through the operating speed range.

As the bearing clearance increases with wear, the volume of oil that is required to pressure the system increases. Eventually the oil pump can't keep up so the idle pressure becomes lower and lower with wear. Heavier weight oil will help get the pressure back up. Problem is, the heavier oil will be much harder to move through the oil galleys, especially the long galley going up to the rockers.

The 15w40 weight oil is a good all around viscosity for the older engines. It's thin enough to circulate when cold, and heavy enough to keep "some" pressure at idle. A good rule of thumb is 10 PSI oil pressure per 1000 RPM. So 15 PSI at idle is plenty sufficient.

I think 15 is fine for hot idle, I fixed the oil pump cover on our C and put 15-40 in it and it has a little more than that now.

Your low oil pressure is due to a little tomuch clearance in the mains , rod and cam bearings , would i worry about it now just yet . Jumping up to 15-40 might get a few more lbs for winter use and for summer use you could try a 20-50 . to make it wright it would take a regrind on the crank and new undersized bearing and new cam bearings .