But I think 1000 years back it would be most useful, to share knowledge about the value of hygiene.

For sure one needs to bring at least the knowledge of the scientific method. This one alone written on a piece of paper and shared with cloisters and whatever people, that have the ability to read, would change the world big times I assume.

One of the biggest hurdles would maby be, to share your knowledge without being identified as magician or witch. Since I do not assume that any king or people of nobility would accept superior knowledge being spread in any other way, then through them.