I have a mate who tells me how you work it: use the replacement oil, write to customs, TELL them, include a cheque, it still works out cheaper, and apparently they cannot process the payment anyway (?!), but you are covered legally if you do that.
Dont know how true that is..
however, there are so many sites that deal with this, that a quick search gives a lot of answers..
In summary, (as I understand it from a look a while back) while some older, simpler and non-turbo diesels (why non-turbo Im not sure, cant see how that would affect it really!) can theoretically run on a good proportion of replacement oil, almost all modern lumps couldnt cope with more than a 5% mix, and even that will degrade all the silicon and rubber seals over time.. which is why official conversions cost loads, they replace every seal with metal ones etc..
I dont think id risk it in mine! and if those details are correct then while i really like the concept, it just doesnt seem worth it!
The little French minicars you see over there, the driving licence free ones with their 500cc diesel lumps, i understand they run very well on it!!