My preference is to change out the oils completely. Napthenic oils, as stated are usually used in freon-type compressors, and are still used in some Ammonia systems even though Group II Ammonia oils provide better performance. The napthenic oils normally have such a low VI that they thin out too much in the high temp zones of the compressor. Aside from basic compatability, you should consider two other items:
1. Are both oils either acidic in nature or basic? If one is basic and the other acidic, in the presence of moisture (from compresion of the air) you will get additive precipitacion.
2. Seals: Naphtenic oils will generally swell seals more than paraffinic oils due to their hier content of aromatic hydrocarbons. In ASTM D 471 tests:
Buna-N seals will generally expand 1.3% in Napthenic, contract 3.6% in Paraffinic, contracting 4.2% when paraffinic is used after naphthenic.
Neoprene seals swell 45.2% in Naphthenic, but only 5.4% in Paraffinic, contracting 15.1% when changing from Napthenic to paraffinic.
The Neoprene seals soften 5.2% in paraffinic, 32.7 % in Napthenic, but harden 22% when replacing naphthenic with parafinnic.
--SO I believe the best time to get rid of Napthenic oils is when you change out the oils and can replace the seals.