So are you saying full fat milk will create a thicker sauce, that could be my answer... Thanks
eyi, each tweet comes in as a new topic/question on food52.com/foodpickle, so the context of your question isn't obvious.
But re bechamel, the thickness will mostly depend on the proportion of roux (flour+butter mixture) to liquid. whole milk and cream will help a bit, but mostly that adds richness and flavor.