I'd first wipe down the fingerboard with some naptha. If that crud is stubborn and won't come off, use an old soft tooth brush with some windex. Once all the crud is off, give the fingerboard the Scratch-X and Zymol treatment. Other than fret wear, it will look like new.
When I bought my used 4001S, the fingerboard was way worse than Ronn's is. It was caked in the same kind of crud that is on Ronn's fingerboard. You couldn't see any wood grain or pattern in it at all. I wish I had taken some "before" pictures. Now it looks like this.
I'm sure Ronn plays the heck out of that bass, that's why it looks like that.