I would go see a Gastro doctor. He can do all sorts of tests to find out what the problem is. You could be lactose, gluten tolerant, etc. I too used to have yellow oily stools, and sometimes still do, but I don't know what causes it. If you can, go to a gastro doctor and if you don't have one have your primary doctor refer you to one.
youve probably tried the usual immodium and what nots have you tried eating stuff like stodgy porridge oats dried toast i nibble cracker type snacks alot dry, mind you seems everyones different hope it helps i heard someone mention a diet of starchy bulk stuff helped them no end it was just plain boiled potatoes rice bread no oils at all its on this forum some where i could be wrong but its all trail and error all the best
go to your doc this can be caused by a lot of different things which can be resolved
it is pointless in guessing here