Frequent O2 Sensor replacment

[LAVATundra]

My borther's 2004 4Runner has 25K and is now on its third O2 sensor (both replaced by warrenty). His local 'yota service center claims he's run bad gas and need to have a $130 "fuel system" cleanout. My brother burns 93 octain name brand fuel and doesn't think he has gotten any bad fuel. My 87 4Runner (286K) and 97 4Runner (84K) have never had a fuel system clean out and never had an O2 replacement. Is the Toyota service man looking for work or is he on the right track? Could the hi-test fuel contribute to the O2 demise?

[Those People]

Does he get gas from the same station?

[LAVATundra]

Doesn't always purchase at same station but buys name brand such as Exxon (usually high test as he claims he gets better mileage).