370 degrees.
That's my "all-round" setting.
HOWEVER, not cooking a part has more to do with TIME then temperature.
If you apply the iron too long, you will start to bake/cook/burn. The parts are designed to be able to handle a certain amount of heat, but too long is not good. You'll probably start to burn the tracks sooner than the parts with those oldies though.....
Important:
Tin your iron before any soldering. Re-tin it every once a while during soldering and remove excess solder from the tip (sponge or "flick" it)
Add a little new soldering tin when trying to get the old solderings to flow. The flux in the new soldering will get the old solder running again (the flux in the old solder has dried out/disappeared over the years). If you don't do this, you can keep the hot tip to the old soldering for "ages" but it _won't_ flow, or will flow very late and poorly.