Mix skim milk and rolled oats in a blender. Blend at high speed for about three minutes until smooth.
Optionally sieve the mixture to remove solids for a smoother drink.
Add sugar to taste and blend briefly to incorporate.
Chill in the freezer for 30 minutes to prevent curdling.
After chilling, stir to ensure it’s smoothly pourable.
Add ice cubes to the mixture and stir gently.
Slowly pour in the juice of limes while stirring constantly.
Serve immediately in tall glasses, garnishing with a slice of lime if desired.