Olive oil seems to be the best bet, but you have to make sure your characters are in a position to have imported oils, since it doesn't grow in Narnia.

I suspect hemp oil would be more readily available in Narnia, but I can't get any good info on its exact consistency.

I'm assured goose-grease was in use in the middle ages; butter is also possible although you wouldn't want to put it near lady-bits.