Steve Lucas wrote:

I've used a few methods over the years to model oily wheel faces on
STMFC's fitted with plain bearing trucks. The first was the old
modeller's standby, Floquil Grimy Black. I then used Polly-S Oily
Black. This I was most satisfied with, when Floquil discontinued this
colour. If only they'd bring it back!!!!

Still shows as available both at Walthers and on the Testors web site.

Bill Dixon