I was responsible for the research, development and production of
mechanical combination locks for the UK government for ten years.
Combination lock wheels and associated parts are manufactured from
brass to very tight tolerances to ensure that lockouts do not occur.

When I took over the job the lubrication method was with graphite
powder, which did work but required annual servicing as the powder
tended to eventually cake and cause the wheels to stick.  Trials
were put in place using WD40 as the wheel lubricant and were entirely
successful, enabling the servicing period to be stretched to five
years.  In all the test period and since there has not been to my
knowledge a safe lockout.  We now use digital locks, but that's
another story.

Malcolm Cole
London