380 sel overheating

Has anybody ever used 20 below or any other water additive because my 1982 380 sel's temp exceeds 90 degrees when sitting at idle for over 5 minutes but once i start riding it goes back down? Oh yeah when it first warms up the temp goes allthe way up to about 115 then rapidly drops down to about 85-90 then it's normal (what's the normal temp?).