Corrosion is a real problem on painted alloy car wheels. Once the corrosion gets under the paint, a leak path results which is the cause of many tyres not maintaining pressure. Bead sealants don't help unless the undermined paint and corrosion are removed first (usually with a wire brush). For this reason I can see why Costco might encourage the use of nitrogen, as it might also inhibit further corrosion of the rim.
I would like to know how the oxygen content of the nitrogen in an aircraft tyre was supposed to be monitored. For practical purposes the only way is to refer to the purity of the nitrogen in the bottle and make an allowance for the air in the tyre before inflation (unless of course it was somehow purged first).