What can cause this?
And what can be used to remove it?

Try oven cleaner then pressure wash them then apply it again and use a wire brush then power wash again.

Looks like years of burnt hot oil caked on the inside. Have it hot tanked at a local machine shop if you have time.

Paint thinner should work too.

Mine had the same thing when I fixed my leaky gasket, I used regular gasoline and it removed everything with a little bit of toothbrush scrubbing...

Thx for all the advice, will try cleaners and work my way to the tank if not.