Diesel Bombers - New thermostat dropped my oil pressure

Hey guys, I need some help. I have an 85 F250 4x4 6.9L. I just bought this truck about a month ago. I have receipts on it that go back to 2001 and it might actually only have 126,000 miles on it. Starts right up and runs like a locomotive. It was running cold so I spent all day today changing the thermostat (had a stuck bolt that ate up some time). The truck is running at normal temp with the new thermostat but when it gets to normal temp the oil pressure gauge drops to zero at idle. If I rev the truck up the gauge will rise, but it'll drop back to zero in idle. The oil pressure was fine before I changed the thermostat when the truck was running cold, and the gauge didn't quiver or act strange at all. Before I changed the thermostat it was blowing a decent amount of white smoke on acceleration. The white smoke is much better now but it'll still blow a puff if I punch it. Would the higher engine temp affect the sender to the gauge somehow or do I have a bigger problem? There's no difference in rpms or the sound of the engine. I just happened to check the gauges again when I parked it after my test drive today or I wouldn't have known the difference. The oil level is good and I'm using 15w-40 diesel oil. Any ideas? I owned a 94 7.3L idi for a few years so I'm familiar with these trucks but have never seen this before. It's my only ride so I need to get this figured out in a hurry. Thanks in advance!