you would only want to consume high gi carbs (dextrose) in your postworkout shake, this is b/c high gi is absorbed more quickly and will create the best insulin spike compared to complex carbs which is what you want postworkout in order to shuttle the protein to where it needs to go as well as restore muscle glycogen.
complex carbs (oatmeal) is the best carb choice to eat any other time of the day other than postworkout, this is because at these times you do not require energy thus you do not want an insulin spike, if you were to get as high as an insulin spike that is created from high gi carbs when you are unactive the quickly absorbed nutrients will not be used and stored as fats. this is why complex carbs are a better choice any time of the day other than postworkout, because they are absorbed more slowly and will not have as much as an effect on blood sugar levels.