1994 Nissan 300ZX I have a Check Engine light on. Car runs

My Question is: For the last 2 weeks the Check Engine light has stayed on and will not go off. I put the OBD1 Computer in Mode 1 and Mode 2. I taped the flashes of the Computer with my camcorder and there are 5 long flashes and 1 short flash. This 1994 300zx is Not a TURBO, it also has California Emissions, Auto trans,AC,PW,PS,CRUISE CONT,Stereo. I recently Replaced the battery with a Nissan OEM battery. I have been running Shell 91 Octane gasoline, but before the problem started I put in some 89 Octane gasoline.The car runs very ROUGH. Could the car have bad gasoline or just too low of Octane.(91 is the highest you can get in Las Vegas, Nevada). Or could one of the Fuel Injectors be bad?
I have pinpointed the flashes on the computer to be a code# 51 which is fuel injector circuit.