Do not forget that the suction port on the oil pump may be real close or at the oil level in the pan. If that is the case it could suck a bit of oil off the surface, enough for a few pounds oil pressure.
The pickup tube is subject to vibration. The tube material and wall schedule combined with threads is likely the problem. A thin schedule tube when threaded will be very thin at the bottom of the thread, leaving very little for strength. A possible solution would be to tape the pump housing while apart to almost bury the tube threads. Make two little triangular gussets out of 1/8" steel. 1/4" to 3/8" ×1". Weld above and below the pickup tube and tack to the housing. This will help alleiviate the pinpoint stress at the pump body and help spread it out over a larger area at lower values. Clean out the housing real well. Pack the pump with Vaseline for startup. It will pick up oil real quickly.
DV has a video on high HP SBF blocks breaking. One area that breaks is the main webs. This is not caused by vertical stresses and vibration, but horizontal vibration of the main caps and webs. Tieing the main caps together with a main girdle changes the natural frequency out of the destructive range. Also cleaning ALL scale out of the cooling system and filling with concrete up to the bottom of the water entry to the block from the water pump changes the natural frequency. On those blocks it is also wise to build a high tensile steel brace to be doweled and bolted across the valley into the oil gallery area. Strengthens another weak area of those blocks.
While working building oilfield compressors, companies would hire specialist companies to come in and test the natural frequencies of vessels and piping. If a 12" or 18" pipe has a natural frequency that is a multiple of the operating RPM, it will eventually break. A change in pipe schedule or welding a piece of steel at a key location like a fishplate would change the weight and natural frequency so it was away from operating RPM multiples. The natural gas engines run at 100% load and throttle for a month or more at a time. Vibration is a constant in the building. When the multiples add up, something is going to fail.
