Euros will use AutoData to cross reference the plugs on your car, AutoData on the whole is accurate but there are some errors that I've noticed when using the system like they state changing 335i Turbo start/finish is 7.3 hours. Which is about 30% of the time needed.
Re the JB4 I'm in expert but can't you clear codes re set adaptations on these things once works completed.
Back to the issue worth checking (though I'm sure you have)that you haven't a weeping rocker cover gasket. Sadly I've had this and a cracker cover both of which caused misfires as oil leaked into the plug wells(strangely cylinder 3 as well as the cover cracked which allowed oil to weep on to injector yes you've guessed it 3.
