I had an auto-tech instructor about twenty five years ago who gave me some great advice. He told me to evacuate and re-charge the A/C system every three years and add about four ounces of refrigeration oil. I've done this to every car I've ever owned and have NEVER had a failure of any part. This may be especially good advice to owners of MB's with suspect evaporators. Here's why. With age, the oil can accumulate moisture. If I remember correctly, the moisture mixed with the refrigerant has a certain amount of acidity to it. When the A/C is not in use, a lot of oil sits in the bottom of the evaporator. This is what causes them to leak. The damage is from the inside out. Those of us who have replaced evaporators almost always find the leak in the same place. On the BOTTOM.