Not sure if people are still suffering with this but I have found a way to clean it off easily.
I purchased a second hand push and it was suffering badly from this (picture attached had hoped to add more but new user restrictions suck).
I tried rubbing alcohol, contact cleaner, other solvents in small test areas and nothing was cutting it. I then had an idea to try Gumption (Australian cleaning paste not sure if this is available elsewhere) and it came off with only a little effort. Given that its a paste be mindful of getting it in ports but other than that it worked a treat!! Also tried some magic erasers and they also worked well. Seriously made a huge difference!! After photos will be in a post below
https://www.gumptioncleanser.com.au/
https://www.mrclean.com/en-us/shop-products/magic-erasers/magic-eraser-original
Hope this help!
Peace Badshore