I was told to use a clean bristle brush and brush the dirt off, then maybe use some water and scrub, but I was mainly told to just brush it off with the thick bristles. I have a black synthetic Abetta! It's pretty comfy so far, but havent been in it for long period of time yet.
Sonny,
I know you can get synthetic saddle cleaner tho I wouldn't bother spending money on it. Depending on what you all dirty just a wet cloth or a brush with a harder brush will also do the trick.