Just remember that all these "opinions" are just that. Personally, my "opinion" is that given that your truck is 7 yrs old, has the earliest itineration of injectors and you ran the wrong oil weight in it may all have combined to cause the issue(s).
I think injector problems are very hit and miss. Some here have terrible problems and others dont ever seem to have any problems so who knows exactly.
Good luck with it anyway. Look on the bright side, you only have 6 more injectors to replace and they'll all be new!!