Is buttermilk preferred for sourdough starter?
Normal sourdough starter is usually just flour and water. However, there are less common alternatives such as using fruit yeast and other sources of yeast. 

The main problem with using things like milk or buttermilk is the danger of causing spoilage.

As another commenter already added, 


  "Adding milk works because milk is mostly water. It might even work
  better, as it also contains sugar (lactose) which will be eaten by
  your starter's yeasts and bacteria.
  
  But it also contains other stuff, such as fats. And fats, after some
  time, get rancid. You probably won't want that taste in bread or
  pancakes. I would stick to water (which quite probably you'll find
  cheaper than milk).
  
  Don't worry about rotten milk: your starter is already rotten. And
  it's a hostile environment for most microbes. It's very very
  improbable new microbes would survive there. But if they do, they'll
  be quite similar to the already existing ones."


A great overview from this comment:


  "A sourdough starter is just Flour + Water + Time
  
  Some add catalysts such as pineapple juice or grape skins etc. But
  what is fermenting is the flour! and this is the starter.
  
  Once the starter is made, however you choose to make one, the finished
  starter will be a flour + water. The added "catalysts" are just to
  help make one. Once it is made then generally it is just fed flour +
  water to be kept going indefinitely.
  
  There is such a thing as Yeast Water which is made and kept going with
  fruit but that is different.
  
  When a starter is used in a sourdough it will be flour + water. The
  temperature and feeding schedule will affect the starter either making
  it more yeast or bacterial therefore making it more or less sour. But
  what also effects the final bread is how the starter is used within
  the dough, i.e. how much starter to use, how long the dough is
  fermented and at what temperature.
  
  So many factors involved here.
  
  No two people have exactly the same starter. And one starter can bring
  out different qualities in different breads depending on how it is
  used. And a change in feeding can change your starter.
  
  Welcome to sourdough."


Best practices from Colorado State University:


  "Considerations for bread starter variations: Amish Friendship Bread:
  Although rare, there have been incidents of foodborne illness
  associated with friendship-style bread, which differs from traditional
  sourdough by addition of milk and sugar, and thus provides an
  environment which could support growth of harmful microorganisms. To
  limit introductions of harmful bacteria in friendship bread starter,
  use pasteurized milk or cultured (soured) dairy and ferment in a
  refrigerator, not at room temperature."


Signs your starter may have gone wrong:


  "Signs of Contamination A starter should be white, light gray or light
  tan. It should smell like bread dough, of yeast or of its ingredients.
  It should bubble subtly and occasionally burp. If the starter has
  liquid on top of it -- this is called hooch, and it is the alcohol the
  starter’s yeast produces from fermentation -- it should be clear,
  white, light gray or light brown. If the starter or its hooch is pink,
  green or dark brown, discard the starter. If it smells or looks moldy,
  discard the starter. If the starter is fizzing or the starter has
  spots or patches -- which are signs of foreign bacterial growth --
  discard the starter."