Don:
I'm no expert but I think 100% argon is good for both. I just checked my tanks and they say Argon. Doesn't say anything on the tank label about argon mix. I use the same tank for chrommoly, aluminum & mild steel. The only change i make for aluminum is the Tungsten Electrode & the polarity Setting. When I get the tanks refilled (exchanged) I ask for argon. Welds look good and haven't failed so I must be close to right.