[Xy-pic] Use special backend

[Carola Werner]

Hello,

I always get this message:

Xy-pic Warning: The linewidth 3.99979pt effect is not implemented with 
the current driver. [xypic:24].

when I try to make the lines thicker.

I use
\usepackage[all,ps]{xy}

So, how do I implement the postscript backend?
with
\special{postscript}??
and where does it go to?

thank you in advance.

carola