This was the response I got from Kamco Products:
The product is color coded and marked as fuel oil and that is its intended use.
If the installation was made using ½” material, it would, technically, meet the ASTM B-88 (Type L) code for potable water, as that is what we manufacture the copper to.
3/8” OD Oil Pro-Tec is made to refrigeration standard ASTM B-280, which is cleaned to a different specification and not designed for potable water. You would have to do some real digging for the suitability of using refrigeration grade for potable water application, if 3/8” OD was utilized. As a rule of thumb, we do not endorse that practice.
However, from a content identification standpoint (and it might be moot being found under a sink), probably not much danger in mistaken line identification as it would be pretty clear where its connected to and what the contents would be. If those lines continue throughout the house, into the basement etc., might be a good idea to apply stickers or some kind of identification that the line is for water, and not fuel oil.
The real issue is substituting gas lines for potable water, or vice-versa.