Since the lack of a Oil Tanker Wagon on pak.96comic I created this one:
(http://img854.imageshack.us/img854/5014/tankers.png)
It's simple, but at least now I can transport petroleum with a train that looks like a tanker.
Player Color compatible.
Edit:Sources now available at 96comic Repository (http://forum.simutrans.com/index.php?topic=10864.0)
For a few days, I have painted a oil carriage, too.