The selenium split-toning process is one that absorbed me for some time and led to my learning a lot about printing papers and techniques. The effect is partly dependent on the photographic paper used - or more specifically its silver halide composition. A chlorobromide emulsion (warm tone) always worked best for me. I found that Agfa Record Rapid was one of the best, sadly no longer manufactured.
As my version of split-toning uses a selenium salt sloshing around in a dish it would probably be banned under health and safety rules these days. No two prints are ever exactly the same - unless you do it with PhotoShop instead (far less toxic but nowhere near as much fun). There is a good introduction to the split-toning process on Jonathan Bailey's excellent site.
These were onions grown in the garden and drying in the shed.
Mamiya C330F, 120 Tri-X, printed on Agfa Record Rapid, split-toned in Kodak Selenium Toner