- Paid upgrades (I.e. cash upgrades), for me have always gone into Z, no matter the cost of them. I have always earned Z Miles & Tier Points for these too.
- Miles upgrades go into G and you usually have to chase for the underlying fare Miles/Tier Points - although I think the computer has become smarter now and should reward the underlying fare.
- Op-Up's go into G, but one time I had an outstation do it wrong and they put it in Z.
Happy to be corrected here, but this is based on my experience of upgrade hunting, quite extensively.