I thought that Stillwater was entirely made up by Cameron Crowe for the movie Almost Famous. But apparently there really was a Stillwater whose music Crowe licensed.
According to Wikipedia (https://en.wikipedia.org/wiki/Stillwater), however, there was a real 1970s band called Stillwater whose works Crowe licensed for the movie, but ended up not using. Instead, the "Stillwater" music was written by Nancy Wilson.
So, looking at the movie soundtrack, there's a song called "Fever Dog" credited to Stillwater" and composed by Russell Hammond (the character played by Billy Crudup).
Who really did play "Fever Dog" for the soundtrack?
I find it hard to believe that it was the actors in the movie. And was it composed by Nancy Wilson or not?
Pulled From IMDb's credits:
Jon Bayless .... additional musician
Danny Bramson .... music supervisor
James Eric .... music playback operator
Peter Frampton .... additional musician
Todd Homme .... executive in charge of music
Carlton Kaller .... music editor
Gordon Kennedy .... additional musician
Mike McCready .... additional musician
Gary Raymond .... music playback mixer
Ben Smith .... additional musician
Nancy Wilson .... music producer: original songs
Peter Frampton .... musician: Stillwater music (uncredited)
Bud Raymond .... music playback assistant (uncredited)
Greg Townley .... music engineer (uncredited)
Elsewhere, IMDb says Marti Friedriksen was Jason Lee's singing voice. Nancy Wilson gets the sole "original music" credit.