1st Gen Zero oil pressure only at idle.....then zero pressure at all times after oil/filter change?

2005 R53, 189k, for the first time ever sitting at a light on the way home after a 2 hour drive, the check oil light came on accompanied by zero oil pressure but only at idle. The pressure stayed up nicely if I just kept 2000+ rpm up. After reading some forum threads with the similar issue I changed the oil Mobil 1 5w-30 and a quality new filter. Started the car up and now I have zero pressure at all times no matter the rpm? I ordered a new sensor and won't do anything further until I have that changed but wondering what would explain the drop to zero pressure at all times after a fresh oil and filter change?

 

Fixed. I replaced the oil pressure sensor since it was $12 and fairly easy to change. That was not the fix. With the help of people on the forum and watching a youtube video I discovered that the "Oil Filter Housing Drain Plunger Spring" assembly was gone from the bottom of inside of the oil filter housing. You can see it or the lack of it with a mirror. Since I found the old part lying in my gravel driveway the plastic retaining tabs had broken and when I replaced the filter cartridge and drained the housing with my suction hose (it was already loose due to the broken tabs that hold it in place) so it came completely out. Outmotoring sells the part for $29.95. SKU: REP_12345 "Oil Filter Housing Drain Plunger Repair Kit"