Originally Posted by
syseng68k
WD40 is petroleum based, so rots rubber and is also inflammable.
Which is why I only ever use it on connectors with plastic housing (and inside the distributor cap). I use regular switch cleaner on rubber connectors, seal with insulating tape and put Vaseline on the tape.
That said, I think in more modern vehicles the "rubber" on things like HT leads etc. is actually synthetic.