100% Argon for mig welding mild steel

Pure argon will work, but it will give very poor penetration. For sheet metal it might work. It is commonly used for MIG welding aluminum. You could get a small bottle of pure CO2 and jset up a mixer to make your own C25 or C20.

I was always taught that a few % of oxygen was helpful too on stainless. Any comment ?

The problem is that oxygen is only useful in tiny percentages, like 1% to 3%, any more than that will cause real problems in the weld puddle. You would need a FLOW gauge calibrated for oxygen. Trying to add that in a manifold system is almost impossible without a very expensive mixing system. Oxygen has a profound effect on the arc heat. 98% Argon / 2% Oxygen is a REALLY hot gas for Spray process MIG on steel.
Helium is easier to use in tri-mixes, but it has a similar problem of being difficult to mix accurately.
I have blended helium with C25 for welding some difficult pipe fittings that had some inherent porosity. It worked great.
Helium is annoying because you can't buy standard flow guages for just helium. The only ones you can get are Smith ball and tube flow guages with a rotating sleeve on the tube for 4 different gasses. The helium scale is a joke. 1/16" from the bottom of the tube is 10 CFH, 1/8" is 20 CFH. All you can do is guess.
I still use it mixed with argon for TIG or MIG on aluminum, but I have no real idea of what the mix is exactly. Someday I will invest in a expensive lab grade flow guage for helium, but until then I just do my best.
You really have to make sure you close helium bottles tight. Even a tiny leak can drain a tank overnight.
Another hazardous mix is Hydrogen / Argon, which is used for BIG plasma cutters.

John: like you, I'm in the novice category compared to alot of these guys in the forum. I've been stick welding for a couple of years, and picked up my first MIG unit a few months ago. The guy at the welding/gas supply store sold me on an 85% / 15% Argon/CO2 mix. As the other boys pointed out... a little less penetration than CO2, but it allows me to MIG weld together the thinnest of steel. Between my stick and the MIG I've got just about everything I'm into covered. I'll be interested to see what the real experts have to say.
Joe

Thanks for the comments Joe. Yesterday I welded back together a tuned pipe for a small 2-stoke engine. Its been my first real job for the new machine. I had cut it into several sections to refit it to an application other than what it was originally designed for. I'm guessing it about 16 gauge. I've got a little experience with stick welding and some rough repair with flux-cored wire. Maybe it was the 100% argon but the tuned pipe came out great. I was really worried about burning through easily but out of 3 total circumference welds I only burned through once in a very small area. The rest look like adequate penetration and very neat and clean. I took it down to a fabrication shop that does a lot of racing stuff and one of the guys there told me they would never have touched it with a MIG but it looked great. I didn't mention the 100% argon though. The pipe came out so good I'm not even considering dressing up the beads with a grinder, now I'm kinda proud. I'm going to stick with the 100% argon for a while and see how it shakes out for me. I don't really have anything heavier than maybe 1/8" in the near future. I will look into getting some 75/25 Argon/CO2 for my own comparison sake though.
John

I wouldn't touch the beads with a grinder. It puts small cuts on them, giving them a groove to start a failure. Thin stuff does very well on a MIG, you just have to get your heat right, and move fast.
STeve

I have both the 100% argon and the argoshield argon/CO2 mix. I have found that the mix works better on mild steel.

100% will arc, but it will not be pretty.
You want a mix it with a gas which can ionize such as oxygen or CO2.

Without seeing the Smith Ball and tube flow gauge, I am only guessing. But I suspect you could disassemble the gauge and measure the ball. Then look in Mcmaster Carr for a ball of the same diameter but made of lighter material. They do have balls made of plastic. Don't know if that would make the gauge easily read, but it might be worth a shot.
Dan

Thanks to those who replied for sharing their knowledge and experience. I got an 75/25 Arg/CO2 tank and I'm glad I did after spending a few minutes on some scrap.
John