I'd never recommend mixing refrigerants. However I'd fully recommend flushing out all of your old R12 (whatever is left in the system) and fully recharging with Freeze 12, that way you have one type of refridgerant in the system.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver