There is absolutely no proof that Jesus' exact words were lost. We discussed this in our Lenten form last night comparing Scripture vs. Christian legend. Scripture was determined by a bunch of men. God was not in the process. We approach this from a Western time frame, that things can be recorded with veracity, that first thing happened, then another thing happened. Jesus was not raised in the Roman tradition but Eastern. Truth is not confined to written words. Authors who claim apostolic authority were not purposefully lying. Anthrolpogists have studied oral traditions around the globe. These people knew how essential it was to remember every little detail. A range of cultures have legends that have been intact with no change in details for many centuries.
I'd be comfortable if the New York Times and Wall St. Journal documented Jesus' ministry. As a Western trained student, I was told to diminsh oral traditions. It is cultrual imperialism.
People knew what Jesus said, what he did, what others said and did. Why bother to repeat it? Better to tackle fresh problems. John writes pure theology b/c theology became a concern. It was not a concern for Mark, etc. I've always believed that the Gospel requires faith. As Paul wrote, it is folly. Strange belief. I don't know for a fact if Jesus exact words were used but it is a very unimportant point to me. The cross is the central message. What does it all matter without faith in the cross?