Regardless of whether or not the gauge has been damaged from over tightening, you need to get a 10MM line wrench. A line wrench is a special wrench generally used for hydraulic lines. Use the line wrench to hold the stationary bolt while you loosen (or tighten) the disconnect line using a regular 10mm open end wrench. If you do this method, your oil gauge will give you many miles of good service; if not, you will continue to suffer from oil leakage issues.