When that oil pressure light started to flicker, you should have shut down the engine and pulled over at the very first safe place to stop. Failure to do that has most likely led to severe damage to bearings and other friction surfaces in this 14-15 year old engine. "Horrible rattling, knocking noises" are a pretty good confirmation that you have severely damaged the engine, and every time that you start that engine, you are driving coffin nails further into the casket for this car.
Unfortunately, "oil treatment" is not a lubricant. It is a Viscosity Index Improver, which means that it thickens oil. If you had very little oil in your crankcase (the most likely scenario), thickening the oil did not help the situation. If the flickering oil light was the result of a failing oil pump, thickening the oil might have been a temporary aid, but continuing to drive a car with a failing oil pump is one of the best ways to kill an engine.
No matter which of the above scenarios is the case, your present plan of continuing to drive the car is only going to ensure that the engine is beyond repair.
You never told us what you found when you checked the dipstick just before you added that STP-type stuff to the crankcase. What did the dipstick show? Did you actually add oil, or did you just add that STP-type stuff?
Most likely, this engine is already "toast". However, if you want to have any hope of continuing to drive this car at minimal repair costs, you will have it towed to a reputable mechanic, and not continue to drive it.