Chuck the green (pure) tungsten electrodes

It's a long story; so here is the condenced version.

The oxides that alloy all other electrodes do several things, two of which are increasing the melting point of the electrodes and making them better conductors. The oxides perform the function of making the electrodes better "emitters."

Zirconium (brown) have a slightly lower melting point then Thorium (red) or Lanthanum (black & Gold) or Ceria (orange @ white). I prefer Zirtung for low amperage AC aluminum or setups with a water cooled torch

There is lots of discussion/debate as to which oxide is the best for specific applicatons.

Cerium is what Miller recommends for AC aluminum and its a good choice, although for low amperage (below 30) the arc tends to drift up the tip of the electrode even at increased AC frequency.

Thorium electrodes are fine but you MUST be careful not to over-heat them. They will deform in funny ways at the tip. when you see these funny deformations (little balls protruding out of the end of the tungsten and cracks) you should assume that some of that electrode is also transferring across the arc into your aluminum, which is bad for a number of reasons.

Miller has a number of articles on their website about power sources like yours and how to get the most out of them, along with good advice on electrodes and how to prep the tips.

The mfg specifically denounces the use of pure tungsten on the inverter machines. I use 2% ceriated for nearly all my tig welding I have also used 2% lanthanated with fine results. I don't like the thoriated and if aluminum is the prime material you may want to try zirconated. Try the guys at www.diamondground.com for tungsten they are great and know their product lines very well.