How can the formula of WD-40 remain a trade secret?
"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      &lt;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).