i have an ‘02’ vw golf gti, every thirty days or so the coolant light comes on and im having to put engine coolant in, don’t see any leakage under the car, just wondering whats going on, and also whats the best kind of coolant i should be using? was using dex-cool but heard that its not good for your car? is this true? please help 
thanks so much!!!
DexCool isn’t good if air gets entrained in the cooling system. When that happens, the DexCool can turn acidic damaging cooling system components and the engine. A universal type coolant would probably serve you better such as the type Prestone sells.
If you have to keep adding coolant to the system, it’s going somewhere. I you can’t see any external leaking of coolant, then it must be leaking internally into the engine.
Tester
That car calls for G-12 coolant. Using anthing else is not good. You need to have someone check it and find the leak. If you are lucky it might be nothing more than a pinhole in a hose, or it may be a damaged engine.
Be sure to find someone who knows what kind of coolant goes into that car. If they don’t know G-12 keep looking. G-12 is pink.
I don’t think there is anything wrong with Dexcool, but the universal coolants on the market now are so much better. I think you will find that they meet the specs for VW coolants.
A very small leak can be hard to find. It seems that an orange or pink colored coolant leaves a trace that is easier to spot. The yellow and green leave almost no visible trail. Prestone’s universal orange might be your best choice until you find the leak.
The leak could be coming from a hose that is not tight, or overtightend to the point that the rubber has been cut. Don’t overlook the possibility that the overflow tank is cracked or you may just need a new radiator cap. Also check the hose between the radiator and the overflow tank.