if 3s or mechanic is suggesting this is because you had stale fuel then sorry mate they dont know anything about cars,
petrol is good for a few years not months, its dinosour oil anyway, unless you are filling the worst quality petrol worse than irani and it has really really bad addictives nothing will happen to it sitting in a tank.
and when it does eventually go/break down it tends to smell like varnish and then it will want the entire fuel system cleaning out, hi octane would do nothing. your symptoms and o2 sensor diagnosis is related to some else problem definitely not petrol
lol i have 4 cars which are not driven and i fill up petrol not before 4 months, not just 1.5months, one of them is a mercedes so if this was the case then all would have long ago have serious problems when in fact there are none.