I have read the 1839 intro by DL now and it's full of references to obscure music publications from the earliest to just before SMM was published, but very little of actual oral tradition I suspect. That seems to be a big problem for those wanting to study Scots songs from oral tradition; until Greig and Duncan came along the overlap between folksong and artsong was very hazy and of course Burns rewriting everything obscured that distinction even further.