Drop inOil Pressure

I wonder if you can help me find the problem that cause a drop in Oil pressure.

This is a 1995 Cummins 330E installed on my Volvo WG.
2 working days after my last oil change (Chevron 15-40) the truck was running at 45 miles aprox. 1400 rpm. Then I noticed that pressure was +- 32psi, moreover I was surprised when the oil press alarm goes on at a light stop. Immediately I check the oil level which was right but I found the oil to be extremely liquid (considering that the oil is recently changed).
After consulting with locals I change the oil again back to Shell-Rottela 15-40 and filter of course.
Pressure increase until the moment reach the working temp (175~180 F) then it goes down again to 25psi at low rpm. If we raise the rpm yes it goes to 45~50psi (+- 1500rpm).
I check all the pressure test ports according to Cummins manual and all the ports are within the acceptable range, except for what they call signal pipe.

Please give any suggestion
Thanks
Flores

 

Thanks for your usual cooperation and help

 

Just to share with all of you

Finally problem was solved about 2 wks frm now. Problem found to be Injector seal (3 O rings). The spill diesel into the engin.e liquifiying the engine oil minimizing the oil viscocity. How we find it? bcz oil level in deep stick keeps increasing minute after minute.