The .mtl file that comes with an object file definition is very limited in terms of texture and shader instructions. That is not Poser, it is how the .obj format is defined. For example: scaling of textures as used for tiling with 'seamless' textures is not supported. If you want to preserve the look in a saved object has in Poser on reloading into Poser save the materials definition in the library and reload them onto the reloaded object. Alternatively use the material room to copy the shaders and then save the result as a prop.

Other formats, like collada, save more texture info but always only to what the standard supports.