fork oil grade

[malc]

Hi seen in the manual all the spec for quantity and air gap but what grade do i put in my standard ns 400 forks?
if any suggegts atf will be very happy as i have a fair bit of that and being skint lol can do with out buyin any more oil but if needs must then i will
thanks for help in advance anticipation of an answer
Cheers Malc

[Alastair]

ATF is fine