This club rule clearly forbids any substance (petroleum product) on the face of a club to manipulate ball flight.
I am aware that many iron sets offer tips on maintenance of the clubs - i.e. cleaning and coating with a THIN layer of baby oil etc. The same can be said for driver heads and faces, putters, hybrids.
If Johnny applies this baby oil as part of the cleaning process the night before a casual round, but does not apply it maliciously the day of play, is he in violation of 4.2.b?