The milkshakes were changed in the early 80's to reduce the fat content and calories. These McDonald's shakes are how they tasted from the 50's through the 70's. Back then the straw would sometimes collapse because the shakes were so thick. ENJOY!
I suggest getting an Hamilton Beach DRINK-MASTER. It is a consumer version of the multi-mixer's Ray Kroc used to sell to the McDonald brothers. (or... a blender works O.K.)
Combine half of the ingredients for the shake flavor of your choice in a the silver cup that comes in the DRINKMASTER, and mix on high speed until smooth. Pour into a cup. Repeat for shake #2. Or use a blender and mix all ingredients on high speed until smooth. Stop blender, stir and blend again, if necessary to combine ingredients.