Oil pressure problems with a 850

Moin municipality…. My moose makes me worry…. And that’s what I put in an oil pressure manometer there. When I start the car, it has an oil pressure of about 4 bar. However, after about 5 km drive that drops to completely 0 bar. Did I recently make an oil change with 15W 40 oil. Did any of you have this before? Is this a defective oil pump or still a storage damage? I depend on the car and have to drive further distances, and without oil pressure I don’t come there w The Volvo has about 308tsd km down…. Thanks for your help far