I bought STP universal antifreeze / coolant Premixed 50/50 Do I still need to mix this with 50% water
No, do not add water. The fluid is ready to go out of the container. Your car may already have a particular antifreeze in it, particularly if the color is distinctly red, pink, green, etc. (each color denotes a different type). If the color is clear and striking, you should find out if the existing color matches the color of the antifreeze that is specified in your owner’s manual.
If it matches, use that type/brand of antifreeze, and not universal antifreeze. When you mix different types of antifreeze (i.e., different chemistries or additive packages) you degrade the service life of the corrosion inhibitors. If you suspect you already have a mishmash of different coolants in there already, the best thing by far to do is just drain and flush the system (which should be done anyway for a 20 year old car) and install factory specified coolant.
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