Replacing refrigerant oil when you have a partial discharge is - at best - a guess. I'm not sure what the specs are for your model, but most cars hold roughly 8 oz. (237 ml) total capacity refrigerant oil in the whole system. Refrigerant oil is typically sold in pressurized bottles where you can recharge 2 oz. (59 ml) at a time.

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95 E320 Cabriolet, 169K