If you're using Blender, there is a "Triangulate" function in it. That message also happens if you are not exporting from Blender properly. I finally figured out how to "Save" my export settings... uhh... if I can find that... lol
... hmmm
, I cannot find my original image, which was courtesy of kapuhy and his settings, but this is to the best of my remembrance...
Don't forget to name the file appropriately to go with the obj you have, such as NSS_Adua.obj so that it matches what you will use it in, otherwise you will have trouble with other items "linked" to it in the game. To "save" your pre-set, click the little "plus" (+) sign in the upper right next to the "Operator Pre-sets" box. Type a name you want and save it. When you click on it to use it later, it will still just show "Operator Pre-sets" in that box, but all the tick marks you had should show. You might not need the "Normals" ticked if you are not using normals.