Chia seeds as replacement of oil in bakery
According to Pure Goodness:


  Chia gel can substitute for half the butter in most recipes! The food
  will bake the same and taste the same (or better) from the addition of
  the chia gel. All you need to do is divide the amount of butter or oil
  in half, and then use the same amount of chia gel to fill in. [...] Everything from cookies to cakes to muffins, pancakes and
  waffles can be made with chia gel as your butter replacement. 


Other sites bracket this range, recommending 25% replacement of the butter or oil (some indicating that it is very hard to tell the difference at this ratio) to as much as 75% (but that is fairly unusual); 50% seems to be the most common recommendation.

My survey of sites shows that most recommend making Chia Gel with a 1:9 ratio of seeds to liquid (usually water).  The instructions at Health Central are fairly typical:


  Put water in a sealable plastic container and slowly pour seed into
  water while briskly mixing with a wire whisk. This process will avoid
  any clumping of the seed. Wait a couple of minutes, whisk again and
  let stand for 5 to 10 minutes. Whisk again before using or storing in
  refrigerator (Gel will keep up to 2 weeks).