Why? Oil paint manufacturers often use non-compatible ingredients. Since it is difficult to know which do, and which do not, it is better to avoid them unless absolutely certain. But... As for high quality oil paints that do not include unknown substances, they still contain oil. Oil(s) in combination with encaustic at inadequate volume can cause the paint surface to breakdown; resulting in chipping, cracking and complete release of the paint layers. It is important to note that oil and encaustic are compatible, so as long as the ratios are controlled. Ratios are stable as long as the percentage of oil to wax is low or high; for instance the closer the amounts become to being equal the less stable.
What is the alternative? Dry pigments. see How to Make Encaustic Medium and Paint, Paint and Pigment Sticks, or Saponified Wax.
Also a personal note: I tend to work a lot with paint and oil sticks over encaustic which contain very similar properties to the above, and I am torn between whether or not this is truly encaustic. An oil painting or photograph coated with a layer of wax is not classically an encaustic piece. I have felt that encaustic is a medium created as one substance prior to painting; wax, pigment, damar (and oil and turpentine if you wish). Of course there is always room to argue.
