[1795]

Sounds to me like your friend might be miss-associating the additives with the poor performance and is not considering another probable culprit, bad gas (i.e., water in the gas). It is common for underground storage tanks to have some water in them and it typically is not a problem, unless the tank has been freshly filled and the agitation of the new gas coming in mixes the water with the gas. Your friend may have filed up at the wrong time. I do agree with Tim in that a person should probably not be filling up with regular gas in one of these cars; premium would be the gas of choice at the pump.

Being that he burned off 1/4 of a tank and then added fresh gas and saw somewhat of an improvement, suggests there might be water in the gas. I have had this happen before in another car and added a water treatment and kept topping off the tank with fresh gas until the situation went away.