If the top of the Piston crown was detonated, i would agree, but the top of the piston that ive read says its fine, so wouldnt really point to crap fuel. Lack of Lube maywell have caused it, BUT if thats the case, why arent cylinders 2,3,4 like that??.If the other cylinder are showing signs then id be looking at potential bad fuels, but im not so sure imo.
Engine overheat from lack of cooling is possible,(and looks like it from the photo's tbh) cylinder furthest from the water pump would be 1st to cop it from my experiance. Poor Lub would again show(or signs of) on all cylinders not just one.
Again, the damage shown is only minor!, compared to major engine failure which that isnt no way near that stage.
What condition are the Piston and Oil scraper rings like Jonny on the other Pistons?. Signs of jamming or carbon burn past the lowest compression ring?