The key question is whether John had independent knowledge of the Jesus tradition. I think it is very probable that he did. One thing is for sure: John had independent knowledge of the setting in which Jesus lived. Now, it is possible that John got his knowledge of Jesus from the Synoptic Gospels and his knowledge of the setting in some other way, but that seems very unlikely. And anyone who wants to argue for that must explain how John got his knowledge of the setting.
So it is entirely plausible that, for example, John corrected Mark on the Temple cleansing. We don't know for certain that he did but it is a very real possibility. No doubt John chose to portray Jesus in his own way but we can certainly look to John's Gospel for useful historical information.
The key question is whether John had independent knowledge of the Jesus tradition. I think it is very probable that he did. One thing is for sure: John had independent knowledge of the setting in which Jesus lived. Now, it is possible that John got his knowledge of Jesus from the Synoptic Gospels and his knowledge of the setting in some other way, but that seems very unlikely. And anyone who wants to argue for that must explain how John got his knowledge of the setting.
So it is entirely plausible that, for example, John corrected Mark on the Temple cleansing. We don't know for certain that he did but it is a very real possibility. No doubt John chose to portray Jesus in his own way but we can certainly look to John's Gospel for useful historical information.