I have never practiced this however I do know amongst other professional printers in Sydney if the scratch was not on the emulsion side of the film
the printers used to wipe their index finger on their nose and collect the oil from their skin and then wipe the oil into the scratch, I have result of this and yes on certain scratches it can make them disappear. The film was always washed after printing and the client was always NEVER told what had been applied to the negative to hide the scratch.

~Steve
The Lighthouse