An old European teacher once told me Rodinal was designed to work well in Europe's winter, so for it 20Celsius was not normal. He recommended me to use it at around 16Celsius. I think the last time I used it years ago I did my tests at 17C if I remember well.
I liked my results a lot more than the ones I had with several films before, at 20C. Ian Grant came to say then there was no difference at all no matter the temperature used. With all respect to Ian and other members, I don't think that's possible. Some things must change at a very low extreme and also at a very high extreme... I don't care too much about a scientific study, I mean, about doing it myself.
If I ever use Rodinal again (I love its grain) I would use it with ISO100 film, at 16-17C, with very very gentle reduced agitation, say every third minute (not stand nor semi-stand), and exposing at half box speed, trying to minimize its evident midtone compression.