It could be some sort of fat intolerance. Good, the HBT for SIBO is pretty easy so I should go fine for you.
X-rays aren't very accurate at diagnosing constipation. I don't think the oily film on the toliet water is anything to worry about. Have you had your electrolytes checked?
My electrolytes are fine. I'm only concerned because I haven't been eating grease and its still there. I'm also going for an HBT on the 28th to see if I have a small intestinal bacteria overgrowth thats causing my symptoms along with most likely IBS.