what kind of transmission fluid would i need to put in my 1996 cadillac deville

Looks like you'll need to use a Dexron III / Mercon or Dexron VI variety of transmission fluid (available at a major of the auto parts stores, like Pep Boys or Auto Zone). Valvoline and Castrol are 2 manufacturers, but there are others. Here's a link for your reference: http://www.oreillyauto.com/site/c/search/Automatic+Transmission+Fluid+-+Vehicle+Specific/04196/C0207.oap?model=Deville&vi=1025794&year=1996&make=Cadillac