Us N instead of non-portable J.
'J' may be non-portable, but why oh why would you build an index on one machine and the export it to another? (Hint:You wouldn't unless you are naive!)
The reason for using 'J' in preference to 'N', is that it is 'native'. That is, whatever architecture of machine you use it on it uses the format that is natural, and therefore most efficient, for that machine.