My unscientific experiences in no specific order:
First, I think there is a difference in the amount of image-tone change and the strength of the dilution of the toner. I have NOT done definitive tests, but often tone the same print in a different dilution when I'm not getting the image tone I want. Stronger dilutions seem to change the image tone more than weaker ones.
That said, I could maybe be convinced that this is not the case by a test. It should be easy, since most papers arrive at a point where toning is "complete." Toss a couple of identical prints into two different dilutions and just let them sit. Compare the "completes" at the end of an appropriate time.
I question whether treatment in selenium toner that has no marked visible effect (if you can call that toning...) still has a positive effect on permanence. I seem to recall that the "archival" benefits of selenium toning are in direct proportion to a change in image tone, or at least, in density.
Not only do different papers respond to the same toner differently, different developers influence the toning too. Papers that tone rapidly and markedly can end up completely reddish and quite low in contrast. Other papers I've had just increased a bit in density and actually turned cooler in image tone. Finding a paper that responds how you like it in selenium is one of the creative choices involved in printing (for me at least).
I also believe that toning certain papers affects the activity of the toning bath and influences image tone on subsequent prints as well. I've had prints that toned wonderfully not tone satisfactorily after a different paper was toned in the toning bath just prior. The difference was not only in activity, but also in the final image tone... I'm not really sure why this happens, but it has happened enough that I am convinced it is real, and not just exhaustion of the toner.
The usual effect of selenium toning (on papers that respond well) is to first expand the contrast range of the print by increasing the d-max and separations by proportionally increasing the density of the silver image. This means that highlights are also affected, but to a lesser extent than the shadows. At times, makes a larger difference in the feeling of a print. (I tend to print just a bit lighter and less contrasty to compensate for the toning.) After a time, however, the image-tone change reduces the density in the shadow areas as it turns to reddish/brownish.
Viewing light makes a large difference in the perceived image-tone change. Since selenium toning usually adds red of some kind to the print tone, the change is more marked under incandescent lighting with lower color temperature. It is less noticeable in daylight.
Toning is one of the least predictable print controls I have, and tends to be the most subjective.
I'd love to see some more scientific data on this.
Best,
Doremus Scudder
www.DoremusScudder.com