I have the same memory! You could buy uncolored margarine in Wisconsin. It had a food coloring pellet in it and you had to massage the container to turn the margarine yellow. I also remember my family driving down to the border to buy margarine.
One time we were stopped but we didn’t have any oleo. The one time we crossed that border without a couple of cases of oleo.
Felt like a pirate. Lol. Treasure Island writ large in a little boy’s life.