i need help with my first gen w250 1990. when my truck warms up my oil pressure drops due to the viscosity change but i need to know if the drop in pressure i have is normal oncw it is warm, the oil gauge at idle up to about 750-900 rpm(guess cause i have no rpm gauge) my oil pressure needle is between a little out of the lines on the gauge or right on the first line where the gauge starts reading. i change my oil every 4000 miles and i never have any metal in the oil or on the magnetic drain plug. do i have a bad sending unit or is that the normal pressure range. once the rpms come up is goes up where i think it should be(second line in the gauge). thanks for your help. i just do not want to ruin my engine.

 

that is about normal but you can try a manual gage to verify

 

The sending units on these trucks are notorious for failing. As Marty Z said, verify with a master gauge if you can. I don't know what kind of oil you're running, but 4k miles seems like a lot of wasted money to me. Still, your engine will wear unbelievably well so maybe you'll save some in the long run.

 

how long do you guys go before you change your oil? i am working on putting a mechanical oil pressure gauge in it currently. you guys would hate me for how much i baby it (1-3 psi max. but it goes if i need it too pretty well (38 psi is on tap).
thank you for the quick help. i want my cummins to last past my life time and hopefully my future kids take as good of care of it as i do.

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delco 400 15w-40w heavy duty for diesels is the oil i run.

 

7000-10000 miles

 

Thank you again for your help all ya.

 

What Marty said. That's with a good filter though, Fleetguard or Wix being my two preferences. And if you run synthetic oil, you can double the interval between changes. Of course it costs twice as much so you don't gain anything money-wise. But synthetic isn't going to loose it's viscosity over time.
If you really want to look after your engine, invest in a centrifuge!

 

I use synthetic and go about 15000 miles between changes but I change the filter at about 7000. Personally I prefer Baldwin filters

 

My oil pressure is weird too. It takes a little bit for it to come up when i first start it. I have a mechanical gauge and the factory gauge, they're pretty much right there with eachother but i dont understand why it takes 30 seconds for it to read on the gauge. It doesnt make any noises. Anybody know whats wrong?

 

Pressure is created by restriction in the oil/lubrication circuit, in this case by oil trying to fit itself between two bearing surfaces, e.g. crank bearings, rod bearings etc. Which is why you can diagnose if the bearings are worn via oil pressure. Lower pressure numbers tells you there is more space for the oil to flow, i.e. less restriction.
So, to answer your question, it takes a little bit for the oil to get moving through all those parts, especially when it's cold. That's the reason for the delay on your gauge.