When the refridgerant runs through the expansion valve/orfice tube, it will hiss. If it lasts more than about 10 seconds, its likely restricted and will need replacement.
Easiest way to remove excess oil is change out your drier (do not add oil) or remove the compressor, tip it up and allow to drain. Replace only about 1/2-1 20oz drink cap full of oil in the compressor. However after you open the system to the atmosphere, you must pull a 28"Hg+ vacuum on it to remove air and moisture. Also, you cannot have the system open long because the dessicant in the drier will absorb moisture from the atmosphere and make it useless for absorbing any remaining moisture that the vacuum may not have removed.
Most a/c leaks do not leak much oil from the system. Personally, I never add oil, unless I replace a component of the system.
Just letting it out will not remove oil, unless you take a line off or something that will create a high flow situation to suspend and carry the oil out.
If the compressor is overworked by an incorrectly serviced system, it will cause idle issues, especially on small displacement engines