It is very doubtful that the approach you mention will do anything to fix a bad O2 sensor. You should use the specified octane for your car-- any more octane than it takes to prevent pre-detonation is just lining the pockets of the oil industry.
1) read the OBD codes (outlined in the 700/900FAQ, but identical for a 240), and find out which codes are set in memory. There are lookup tables in the FAQ, but you can also report back here.
2) Measure the output of your 02 sensor. It isn't hard-- get a multimeter, set it to the 2VDC scale. Peel back the rubber cover over the sensor line at the firewall (leave it connected). The car has a 3 wire sensor, two wires which are the same color go to one connector at the firewall-- that's the heater wiring, another single wire is the sensor line that provides input to the ECU-- it's the one you want to measure. Warm the car up (drive it around the block), then touch the positive lead to the sensor wire connection the black wire to a good ground under the hood. If the sensor is good, the voltage will swing back and forth between about 0.1 to about 0.85V-- it should switch back and forth roughly once per second, but that will be hard to see with a slow digital meter (watch it for a bunch of cycles and take note of the lowest and highest value you see). There are additional tests in the 700/900FAQ about how to create a rich and a lean state to assess the sensor integrity.
3) Are there any symptoms? Excessive fuel use, black tailpipe, loss of power, hesitation or stumbling? Let us know!
Good luck!
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