My welding supply store warns AGAINST using a soapy water
solution to check for leaks on bottled gases, especially
oxygen...UNLESS one is absolutely certain that it is a NON
PETROLEUM based soap being used such as liquid Ivory. I can't relay
their explanation in detail but the jist of it has to do with the
gases moving into the regulators with such speed/pressure
(especially the oxygen.) that the "friction" generates enough heat
to ignite petroleum residue. They note of course that this is
undesirable since open flame and bottled gases don't mix well.
The problem with oxygen and petroleum based materials is one of
combusiton. You need oxygen to burn materials. Normally you need a
source of ignition. However, under high pressure (such as an oxygen
cylinder) sufficient pressure will result in combustion without a
spark. Oxygen regulators are specially cleaned of oils in the
manufacturing process for this very reason.
As for the petroleum based soap, I doubt that it makes much
difference because every soap I can think of is made of organic
molecules (carbon, hydrogen and oxygen) which are all combustable.
So any soap you can burn with your torch could be a potential source
of fuel. I would guess that when placing a soap solution on the
outside of a leaking gas fitting any leak forces the soap out; no
leak, no soap goes in either. Keeping all the high pressure
fittings relativley clean is is important. I have always used what
ever soap I have handy. Better to know of and fix a leak than to
leak oxygen into a confined area with an open flame present.