I have a 2002 4Runner and my brother and I are changing the timing belt. While detaching the A/C compressor we noticed a hiss at the suction / discharge connection. We tightened to factory specs but there was some oily liquid present coming from loose connections. How do we check the compressor oil level and the refrigerant level and then is it possible for us to purge the air from the system with just basic tools? And finally, is the refrigerant, like the coolant, strickly to be replaced with Toyota brand? Just a note, the A/C seemed to work fine last summer.