This is entirely conjecture on the part of some scholars who are a bit illiterate, imo.
There was no need for this so-called "oral" period of scripture transmission. Jesus' culture was an extremely literate culture, and evidently, one could hardly throw a rock without hitting a scribe. Jesus took the scroll of Isaiah in the synagogue one day. What does that tell us? It says that a carpenter's son was being taught to read in that era.
For us to pretend there was no education, when it's clear that this culture had long treasured their scriptures is a bit disengenuous.
Luke very clearly points out at the beginning of his gospel that he collected the sources available, compiled them in chronological order, and then passed that account of the life of Christ on to his friend Theophilus.
It is common to assume an underlying text or texts were consulted as the foundation for the life of Jesus. These pre-gospel records make perfect sense, since there were a number of scribes who attended Jesus' every move. (It is possible that 7Q5 and its Genessaret text that so closely parallels Mark is a piece of a scribal record, if it is not a piece of Mark itself. That fragment has dated earlier than 50 AD. That's roughly 17 years after Jesus' crucifixion, and one must allow time for its having been penned and saved.)
BTW, Luke's not being an apostle is totally vindicated by his going to sources approved by the apostles to compile his account.
The only reason this oral period is brought up is so doubt can be cast on scriptures. And it is brought up with absolutely no regard for the advanced culture of the time.
Everyone else in the entire Roman empire kept records, made reports, transacted business, conducted trials, and wrote and read extensively.
Except for Christian Jews and Gentiles. They were all out in left field playing the telephone, message-garbling game. /sarcasm.
Except, of course, when it came to such important figures as Jesus Christ, or the slaughter of the innocents by Herod, and other major events (Exodus) described in the Bible and nowehere else.
It is possible that 7Q5 and its Genessaret text that so closely parallels Mark is a piece of a scribal record, if it is not a piece of Mark itself. That fragment has dated earlier than 50 AD
This is an old theory. The work done on 7Q5 was shown to have been the result of researcher's bias. Subsequent computer matches yielded sixteen possible matches, not unique to +Mark.
Cave 7 in Qumran yielded many non-biblical texts. The "gge" fragment that was interpreted as "Gennesaret" can also be part of "eggenesen" (begot) someone's genealogy. The Q75 has so little information, everything about it is practically a conjecture. The picutre is worth a thousand words.
Luke very clearly points out at the beginning of his gospel that he collected the sources available
Well, he didn't collect them from +John for sure. If he was 'inspired' as +Paul says he was, then he wouldn't need a collective memory of others, but would be guided by the Spirit to write word by word, correct?
When Paul makes his confession before Agrippa he is very confident that the events Paul describes are known by him, since it did not happen, as Paul states, 'in a corner'(Acts 26:26)