Hello!
That tractor is lookin' great!
Maybe you could use my experience to do the grease on the fifth wheel:
It's been done by putting some black oil paint, straight, on the fifth wheel and then working that paint while still wet. One of the tools was a piece of styrene sheet with a "kingpin" in the middle that was "connected" to the fifth wheel and rotated a few times - that leaves nice marks on the surface of the "grease".
Hope it helps - good luck with your build and have a nice day
Paweł