degrading interior plastics?

[USSEnterprise]

I have a 1996 SR5. On a lot of the interior plastics, mainly of leather or leatherette seem to degrade when they are wet. This includes the steering wheel (but not the horn/airbag), the 4wd shifter knob, the parking brake handle, and the shift lever (auto). When wet, a black residue gets on my hands.
I have considered the possibility that its some kind of dirt, but I've tried cleaning it to no avail.
Is there anyway to fix or clean this. Next on my list is a steering wheel cover...

 

[004Gunner]

Yeah man that is just seriously dirty and partially degraded rubber/plastic from lack of care. You can definitely improve it by cleaning it but harsh cleaners may just further break it down and make it sticky so I would use something designed for cleaning rubber or plastic. The combination of greasy/dirty hands mixed with oils really goes to work on an interior. Before you put on a wheel cover, clean it several times to remove as much grime as possible. Plan on taking a couple of days at least to work at it because you will probably not get all of it off in one shot. I've used armor all cleaning wipes and they work great for mild grime removal but I don't know how well they work on heavy grime. Don't use goo gone or goof off because it is too harsh and will smear around whatever you are trying to remove. Windex doesn't work either fyi. Good luck!