Wash chicken breast and season liberally with salt and pepper on all sides. Heat the oil in a skillet over medium-high and and place two chicken breast at a time on the hot oil. Let it brown slightly  on both sides, but don't let it cook all the way through. Remove from heat. Repeat this step with the remaining two chicken breasts. 

Layer the bottom of the crock pot with a thin layer of salsa and place the chicken breasts on top. Season with taco seasoning mix and add the remaining salsa. Give it a little stir to blend the seasoning with the salsa. 

Cook on high for 4-6 hours or on low for 6-8 hours. When done, the chicken should shred easily when stirred with a fork. 

Serve on a tortilla with your favourite taco toppings or on a bed of rice and vegetables.