"How can it remain secret?". Trivial answer is - it can't (it isn't).
WD40 is a Fast Moving Consumer Good (FMCG). It is also a reasonably cheap product - as an industrial chemist this suggests at least two things to me:
- It doesn't contain anything rare or expensive
- It isn't difficult to manufacture
As an ex-petroleum chemical analyst, with access to a reasonably complete lab, checks to do :
Get the Safety Data Sheet (below is composite information from a number of WD-40 Penetrant SDS)
cas %
64742-47-8 50-70 isoparaffins petroleum hydrotreated HFP
64742-56-9 30-35 paraffinic distillate, light, solvent-dewaxed
61789-86-4 <5 calcium petroleum sulfonate
111-76-2 1-3 ethylene glycol monobutyl ether
Not Available 1 fragrance
Check the SDS info using physical/chemical testing
If you have the gear available :
Density
Distillation
FTIR
GC (GC-MS if you have it)
AAS/ICP - metals (ICP multielement would be best)
CHOSNP analysis for hetero atoms
Rebuild the product from data. Check to see if it performs the same.
Improve on the product (believe me - WD-40 is NOT the be all and end all of penetrating oils - technology has improved since its formulation).