The only way to "service" the automotive a/c system would be to completely disassemble, flush all components, dump oil out of compressor, rinse compressor with fresh oil. Replace all o-rings and filter/drier, evacuate and charge.

The risk of introducing moisture into the system and the risk of a leak due to this process, FAR outweighs any possible gain.

If it is cooling well, and has lost no more than 1 oz. of refrigerant per year, count your lucky stars and be happy.

This is a perfect situation in which to apply the old addage "if it ain't broke, don't fix it."

Good luck,