I do a fair amount of business with NAPA and that's where I get CRC branded Mass Air Sensor cleaner. I'm sure that Autozone and others will have the same or similar products. MAF systems are fairly delicate and you don't want to use brake, carb or any other cleaner not specific to the job.
The 'important' O2's are the ones before the cats since they have the A/F reporting job. The ones after cats basically only tell the computer that the cats are doing their job. An O2 would be fairly easy to spin in the bung if they had been replaced within the past 20K or so, given the miles on the odometer you have. If hard to remove and they appear ancient and crusty on the exterior, then they are. If you pull one and there's anti-seize on the threads, then it's a replacement, but without some other info, you won't know when. If you remove one from before a cat and it's sooty, then it's another sign of running rich. Vacuum leaks can also cause the rich running, which in turn screws up the O2s. If the sensor of the O2 has what looks like a silver gray fingerprint on the sensor projection...it's shot. That means someone touched it during installation...a no no.
A black or brown crust would indicate oil contamination....which will destroy the O2 performance in a heartbeat. Usually, but not always, the cause for that is a bad PCV valve. I pull those and replace at 40K since they're so cheap.