AIX

Why should you? Again we are talking Unix here. Use sth. like
# tar ... | dd of=/dev/rmt0 obs=20
It would be just a matter of combination to write directly onto a tape.
You might find the following information useful:
http://www.ahinc.com/aix/backup.htm

Thank you for the info.

1) Looking at the link you provided, I see something which puzzles me:
I don't understand this as we have one weekly backup done with TAR that spawns on 2 tapes !

So this makes me wonder about the truth of the following:
2) For our TARing to to 8mm 5Gb tapes, we use a blksize=10240. What would be the blksize used for DD and GZIP v1.2.4?

3) Humm, I see that now all our servers have GZIP installed. Is this 1.2.4 version a free one ?

It might be necessary to distinguish between programs that have the same name but originate from different sources. AIX tar is different from the Gnu tar (from the Linux Toolbox). AIX dd is different from the Gnu dd.... I think you got the message. So when you look for information that is specific you might look up the IBM Information Center for AIX programs and somewhere else for Gnu programs. Don't compare Linux to AIX. Many things are alike but when it comes to the details it is pretty different. AIX is not Linux.
Last thought: AIX dd supports spanning across several media since AIX 5L (span=yes) came to market in 2000 or 2001 ....