As a Highlander, born and bred, I use a mixture of pinhead and medium-cut oatmeal, water and a good amount of salt. I'm afraid that I cannot offer any exact measurements. After many years of experience, I know what's right for me, which is a thick, well-seasoned mass. Bring to the boil, stirring constantly, in a pan on the top of the stove, then simmer - still stirring - for as long as it takes to achieve the texture you want. Ignore all folklore about having to use a spirtle/spurtle and stirring clockwise etc. Eat from a bowl, with cold full milk. On high-days and holidays only, a small trickle of golden syrup on top is permissible (and delicious).
My maternal grandfather was one of those people who enjoyed making everything into a ritual. He had his (equally thick) porridge served in one bowl, with the cold milk in another. Starting from a position by the rim of the bowl, he would take a spoonful of porridge and dip it into the milk on its way to his mouth. The following spoonful would also come from a position by the rim, adjacent to the hole left by the preceding excavation, and so on: working around the rim and then inwards in a spiral towards the final, central blob - after which the dregs of milk would be drunk, with much lip-smacking and expressions of satisfaction. Mesmerising to experience! As a thoroughly modern adherent to 21st Century mores, I abjure a separate milk bowl, but I do find myself inexorably spiralling inwards, adding small amounts of milk as I go....