I just found this bulletin:
http://bethnrayndogs.com/car_docs/Porsche/porsche/Porsche_Tech/Porsche_TECHNICAL_BOOKS/Tech_Bulletins/Gp3_4a-99-1eps.pdf
The Shell 999 917 546 00 seems to be the Porsche Oil, at least the part number matches.
The others are all in 20 liter sizes
Doing a search for Shell 99991754600 leads me to a lot of threads, but no places to buy.
For the amount of time I have spent researching this I have probably used up any savings from doing the job myself
Steve