"Limp Mode" & service engine light

BangZ

Hi Z peeps,
I have an '09 Nismo and hit this "Limp Mode" people been talking about. Also my service engine light has come on. This all started right after I replaced the original battery. Right after I did that the Service Engine Light came on and will flash from time to time. Driving it to the shop is when I hit that Limp Mode at 3500 rpms (I just got gas (93 octan) before this too), the guy ran the engine code p0300. It's not the O2 sensor..The guy said its sounds like a misfire in one of the cylinders and it idles kind of rough, he spent about 2 hours going all the basic stuff and couldn't figure it out which is weird cause the car only has 36,000 miles on it. so before him pulling the engine apart I wanted to look through these threads and but couldn't really find a good answer for these problems cause Z's love jerking us around with how glitchy they can be but if anyone could give some knowledge on these headaches or had a similar problem and they fixed it, It'd be much appreciated. Thanks!

Also could the Limp Mode be cause by a rare occurrence of getting bad gas from the gas station? Again I always use premium 93.