well , lpg in spain is finally a practical proposition
problem is on the A8/S8 you need not only the correct conversion but a converter who knows the intricacies of converting a multi cylinder aluminium car
http://www.go-lpg.co.uk/Audi_S8.html
this company are known for A8 conversions in uk and use the Romano system as you can read ; the Prins conversion also has a good reputation
http://www.lpgforum.co.uk/index.php
this is an excellent forum for advice on lpg ;rossko is particularly knowledgable