Help me RB26dett low oil pressure issues bad oil pump?

Help me figure out what to do for my RB26dett low oil pressure. RB26 oil starvation trouble shooting why I have low oil pressure with my Nissan Skyline GTR. My RB26dett crankshaft is early production with short snout. I oil pressure has been low since I bought my car over 4 years ago. I will change the engine oil with and new OEM Nissan oil filter on my rb26dett and start the engine. I replaced the oil pressure sending unit and factory oil pressure gauge reads about the same as the Omari factory aftermarket gauge. Have about 3 bar under max load and higher revs. The oil pressure should rise higher according to the factory service manual. Have factory oil cooler and no aftermarket oil cooler. I believe my oil pump in my beloved Skyline R32 GTR is having issues. Let me know what you guys think.
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