Dont bother with pure argon.
Standard Argoshield is fine.
Ages ago I had a serious look at this one, and was offered different gas mixes to try by my local supplier. You go get a bit more spatter and it sticks really badly.
The best way to deal with this is shield ares around the weld with heavy tin foil and use anti spatter spray. Will see if I can dredge up my original post.
Original post, on page 2 there are some photos of the effects using different gas mixes and an explanation by Technic Al.
The apropriate grade of Stainless wire and a gas mix with 95/98%Argon 2/5%CO2. See here
As said, cougar5 (93%Argon 5% CO2 2%02) will work, but after looking through the gas reference chart, Air products' Inomaxx2 (98%Argon 2% CO2) would give you better results, depending on the thickness of the material. I guess it just depends what you're doing, and whether blackening of the welds is an issue.
You dont need to change anything unless weld corrosion is an issue, and if it is you need to add the cost of stainless wire to what you are charging for the job.
Let me try my link again. http://www.mig-welding.co.uk/forum/showthread.php?t=19083
I did get hold of a mix of98% Argon/C02 to try.
Horrible, lots of spatter, the welds were very raised. the one far right in the pic on page 2, cant upload straight to here as I deleted the pic from my pC