99 Cummins low oil pressure
 

I just noticed my oil pressure is reading lower than normal, the needle is steady at the 10o'clock position on the stock guage which is in the middle between 0 and 40. Oil pressure is steady at this reading, does not fluctuate at higher RPM's?

These gauges are notorious for not being accurate is one problem here. The oil pressure is also affected by viscosity levels which is why when it's cold outside you will see higher pressure then when it's hot out. It may be comin on time for an oil change and your oil is breaking down and thinning out.

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Truck only has 89K miles on it. I have ~2K miles on this oil change (AMSOIL 15w40 Synthetic). This is the first oil change on synthetic. The pressure has always been >40.

The fact that this happened with the synthetic change pretty much explains it. Synthetic is thinner then dyno oil in the same weight. It flows better because it doesn't have to be so thick to work even better then conventional.

Pressure is nothin more then a measure of resistance and the thinner oil has less resistance moving it through the system. Sounds to me like all is fine here as long as your not losing oil.

X 2 what Bubba said, I went to some Rotella synthetic instead of my usual straight 15w-40 Rotella and my pressure dropped also.

Thanks for the responses. I'm still a little sceptical. I changed my other two vehicles (Jeep Wrangler and Cherokee) to AMSOIL synthetic at the same time and see no difference in their oil pressure. The truck does run and sound the same with the low pressure reading, no noticeable difference.

Check your oil for the smell/texture of diesel fuel. Also try to confirm the pressure readings with a mechanical oil pressure test gauge.

What kid of pressure guage are you running? The stock OEM guages in most vehicles are nothing more than idiot guages.

I have a mechanical oil pressure guage on my truck and I can assure you synthetic oils (even 15W40 syn) register less oil pressure than dino oils.

In the past Ive ran 0-w40 syn in the winter and if I dont change it in the summer, my oil pressure sender goes off and Im only at 30 psi oil pressure at idle. Change the oil and put conventional oil back in and everything is fine.

What Doc Evil said - my understanding is that the stock OP gauge is tied to software in the ECM that models pressure on factors like engine temperature and rpm's. So if you're stock, it may indicate an issue with an entirely different engine parameter.

If you've got a real gauge, then one possibility not mentioned here is one or more damaged camshaft bearings, which (in my experience, unfortunately) will cause lower but steady oil readings in the Cummins engine. But that's a "worst case" - I agree that the syn oil would more likely make the difference.