300E A/C Repair, what to do with oil?

[John5788]

I have 2 300Es (one's a 300CE, still M103) that have have AC systems that sound like they are working but just have no refrigerant. They are still using R12 fittings. As far as I know, they have not been charged for over 10 years, both vehicles.

My question is, what should I do with the oil that is in the system? I don't even know what is in there, so should I just dump it all out/flush the system with one of these kits:

http://www.amazon.com/FJC-2710-System-Flush-Kit/dp/B0002JMEQW/ref=sr_1_1?ie=UTF8&qid=1440799964&sr=8-1&keywords=ac+flush

If I flush it out, what should I replace it with? Does it depend on what refrigerant I am going to use or does it depend more on the compressor I am using? I am reading a mix of ester oil, PAG, and mineral oil when I am going through posts from this forum regarding A/C oil.

Or is it alright for me to assume the oil is fine since the compressor hasn't seized or anything and just vacuum/charge it with refrigerant?