S/H seats from where you got your pump from?

I think it may have been in this newsgroup that cat litter was suggested
as a way to help get oil out of a cement or concrete drive.  That would
probably help get it out of your car, too - even the fabrics may give up
at least some of the oil eventually.

Google comes up with some suggestions for

   dry-clean oil stain

and

   diesel upholstery stain

These look interesting <http://www.shoutitout.com/oil-stains/> (commercial
product) and <http://www.thriftyfun.com/tf868020.tip.html> (a web forum
discussion on this same sort of problem).

Halfords etc may have products that could help too - but the 'Listerene'
suggestion in the web forum above is intriguing.

I've used 'white spirit' to get tar and grease off the car paintwork - but
the fumes from that are worse than from diesel oil, and it may not be good
for fabrics.  

Many years ago, I remember my mother using eucalyptus oil to clean the
'beach tar' from clothes and picnic things (back in the days when ships
freely dumped used oil and gunge into the sea and it wound up as more or
less sticky lumps on beaches where kids played).  At least it smells nice
(but I think it's poisonous too, in large quantities).

Hi Steve,
    a very cheap and efficient solution that is really worth a try is the
baking soda (aka Bicarbonate of Soda). Doesn't leave any stains.
Let me know how it goes!
Regards
    Xserty