Never seen this before. Chevy oil pressure problem

I see something new everyday I installed a used (Previously Reman) 4.3 vortec engine into a chevrolet s10 pickup truck. After the installation, I noticed a strange issue with the oil pressure gauge dropping to zero at idle once the engine heats up. My Favorite Tools!! Kingbolen K10 Pro Scanner https://amzn.to/4uLhIdo Cyclops Light https://amzn.to/4mOV92c Perfect panel trim tool https://amzn.to/3WoL0yc Oil Filter Cutter https://amzn.to/3IIqaH4 1/2 Cut Stubby Torx https://amzn.to/42oYgqf Get vidIQ to grow your channel faster! 🚀 https://vidiq.com/WrenchingWithKen