A drop in oil pressure at idle as the engine heats up is normal. What you should be concern with is if the oil pressure goes back up as soon as you rev it a bit. Different engines react differently. As long as you have good pressure when not at idle and the pressure is stable and never gets below 20 psi at idle, you'll be ok. This is a common problem on engines that have grooved bearings or the oiling system has been modified by making oil passages larger. Maybe even if the crank has been cross drilled or grooved. Just to be safe, remove your oil filter and open it up to make sure you don't have metal particles in it. If you do, it's most likely babbit from your bearings. You will have to take your engine back apart and check it for clearances and damage to hard parts. JMO George