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?).

Your temperatures sound normal except for the t-stat opening temp. The 380 t-stats were notorious for sticking. We used to drill small holes in the valve and cut of the temp feeler. The point was to get the t-stat bulb to heat-up quickly enough to avoid the spike temp.

I would replace the thermostat.

I believe your rad fans are electric. Your temp goes up because there is no flow of air while the car sits. At about 110C your rad fan kicks in and cools the rad, thence the temp drops. perfectly normal.

308sel overheating
 

Mercedes suggests not running any additives to a proper coolant mix, your running temp looks normal. When was the last time you changed the thermostat or flushed and renewed the coolant and checked the pressure of the resovoir cap, all easy things to do.