(source: Iceberg Slim - Pimp, The story of my life)
the section that contains the expression:
"My four whores been humping at the Franklin since early this morning. I can't miss a five G score for the three days even with Angelo's thirty percent off the Top. Ain't but a C a day for a girl in oil for the heat."
Glass Top (a senior pimp) explains to Slim where and how his girls are working that night. He even tells his previous calculations about how much money he will earn with that.
'C' surely means 100 $ here, but what is 'for a girl in oil for the heat'?