I don't dispense beer with food grade CO2 either.
Gas is gas.
If the tank has rust inside, it won't infect the beer, but could taste funny.
I don't filter my disposible O2 gas, and never had an issue.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10