purging and pressuring ac with n2

hi every one
i want to use n2 in purging and testing ac for leaks
and first time to me
my q are
1_ what precautions i must make in care when use n2 in purging etc....
2_ which type of n2 i must use
because in iraq we have pure n2 only but some experts adviced me to use impure n2 cylinder which contain 90% n2 and 10% oil because its best than pure n2 in purging and testing