Oil pressure light coming on at idle

First off, per the 1966 Owners Manual this is a "normal" condition. But its certainly not optimal. Pull the valve cover. Is the engine a sludge monster? This will tell you a lot about how a slant was cared for. It could be that the oil pressure switch is sludged up and idle pressure won't turn the lamp off. A bit of crud may have the bypass stuck part way open too. Actually pretty common on grandma driven slants. Clean the bypass, replace the switch and run 15w40 Rotella. Also run the engine with the valve cover removed. Make sure everything is getting oiled. I've seen slants where the rocker arms and shaft were so clogged up that the rockers actually squeaked.