I need to add coolant to my 01' SLS. It has the pink stuff I think, in the auto parts store they have a Peak or Prestone that says you can add it to any kind you have in your car already. Do I just buy it and add the amount I need? I am a little nervous about adding a different anti-freeze, but I don't know what type I already have. Thanks in advance

Use Dex-Cool. Premix it 50/50 with distilled water before you top it off (or buy it premixed).

it is a sealed system. If you have to add, you may have a leak.

Your factory fill coolant is Dex-Cool.

There are several coolant producers that are licensed by GM to manufacture and package Dex-Cool. As of May, 2006 the list included:
Chevron (which includes Texaco)
Prestone
Shell
Valvoline

If the container does not say "Dex-Cool", do not use it!