fuel troubles

a few weeks ago I accidentaly put plus 89 octane in my 95 svx instead of 93 octane premium. I was at mobil who puts their premium in the center of the three pumps where every other gas station puts their plus. and stupidly i wasn't looking at the octane rating and put in the gas on the end, where the premium usually is. Ever since then my car has trouble starting up cold first thing in the morning (which has been getting better after about 4 or 5 tanks of premium), and makes a weird rumbing sound for the first 5 minutes, and if i press the accelerator the engine stalls. Besides that the car runs fine. The other problem is that the check engine light has been on ever since and does not go off. Is the problem that the oxygen sensors need replacing? At the motorsport warehouse site it said in the product description that bad oxygen sensors would cause the check engine light to go on, and that they would need to be replaced every 80k miles, which my car just hit.