Best oil for my 2002 BMW 325 CI

I  just bough a 2002 BMW 325 CI with 160,000
miles ...  It doesn't smoke at all!  From a cold start,
while driving,  or just idling ... no smoke at all ! I am
the third owner and want to make sure I put the
best oil in it to keep it running as well as it is ...  I
was told to use Castrol oil 40wt but also  have
heard many others say that they're  using  mobile 1   
Oil not sure of the .(wt) ...  both been synthetic  .

I'm the third car owner, and the second owner, she
had no idea what Oil was used in it.

Recommendations for what oil I should use ?

Thanks