Milk!!
Thats what you need, milk.
Pick a really hot day and put down the equivalent Gold Top Milk to oil spilt. Do up all the windows and leave with the engine running and the heaters on full for as long as possible - leave the car for a good few days and Hey Presto!!
OK you wont have got rid of the oil but you will be so ****ed off by the smell that you will forget all about the oil spill!!
Seriously mate, thats a real **** of a job, I have done similar (2 litres of two stroke) with my Renault Scenic - i ended up taking out the carpet and cleaning it in my local Launderette (wasnt going to risk trashing MrsOutlanders new Zanussi!) and I had to replace much of the high density foam with aftermarket stuff - turned out to be much quieter afterwards tho';-)