If you opened the system and flushed the condenser then you probably lost oil in the vented gas. You should pull and drain the compressor, then refill. I agree with birdchaser. Ester oil is best for a conversion. I upgraded my whole system to a new Nippondenso 6P compressor, so it was empty. The 6P needs 7 oz. total of Ester oil. Once it was all ready, I put 1/2 the oil into the compressor and 1/2 into the receiver/drier. I used R12A, so there was no need to vacuum. I did anyway to confirm it was sealed, but released the vacuum before filling. The conversion works awesome.
Jose