Pls. Help diagnose low oil pressure problem

I bought an '89 Dodge B250 Van with the 5.9 L gasoline engine (throttle body fuel injection). It's got 123k on it.
The vehicle drives and sounds great, starts right up even after sitting,
doesn't use any oil -- only problem was that after driving it a while, the oil pressure would drip way down and the van would stall.
So, I replaced the oil pressure sender, then the oil pump. No change. I even installed a good mechanical oil pressure gauge.
When I first start it (and the viscosity is high), it's right up around 60 psi. After I'm driving for 15 minutes or so, it's down to 40. If I come to a stop at that point, it will drop to 20. Drive it a bit longer and come to a stop, and it falls right down to zero.
At this point I'm guessing -- crankshaft? What would cause that?
Related question -- can that be done from the bottom without pulling the engine? I was planning on replacing the timing chain anyway, so it's not that much more a big deal to pull the transmission--I just don't want to do the job and discover it's something else entirely.
Thanks.
Joe
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"Exotic-scales" wrote:

change.
A few things to do here. One is to check your cooling system because the warmer the engine, to lower the oil pressure it will have especailly when old. Secondly, if you are using 5w30 it STOP using it as it is not a good choice in a worn engine especailly in hot weather. I would suggest that you use 15w40 (not 10w40) in hot weather and you will see a 10 to 15 PSI boost or more in hot oil pressure. During winter use 10w30 in it. Also change yor oil more often in a worn engine, maybe every 2500 miles tops because there is more blow by and oil filter gets dirty quicker which can restrict oil flow and pressure and while the filter will evenually bypass it will lower flow and pressure some in process and also recirculate grit and increase engine wear.
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Thanks for those tips,
I was actually thinking I would run it with Mobil1 15W50 & see if that helped it any. I may try putting a lower temp thermostat in it, though it currently runs a notch above center. I've never owned anything with this engine in it before, so I don't have a good feel for what' normal. My prior (a beloved '80 short bed with a 225 slant 6 & 4 speed od stick shift) usually ran a bit cooler.
Joe
Joe

http://www.autoforumz.com/Dodge-Pls-Help-diagnose-low-oil-pressure-problem-ftopict132607.html
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My Dodge owner's manual says, "never use 10W-40 oil in this engine." The two recommended oils are 10W-30 and straight 30W. Check your manual.
Al
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wrote:

When you get over 150,000km you throw the manual away, or you throw the car away.
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