Spoke to the dealer I am buying from. They also said about $380 because a normal B service (15,000 km) is about $420 and all they do there is change oil and filter plus plug in the computer diagnostics.
So I though I'd check out the price of oil.
My dealer says Ford has stipulated for GT Mustang Castrol Magnatech Professional 5W20. I could not find that on a retail sight so I called Castrol. They said 5W20 is not available in Australia so Ford has authorized Castrol to substitute that with Castrol Magnatech Professional A55W30. Can be purchased in bulk:
20 lt code 3369740 $139.26 plus GST
205 lt code 3374636 $1,343.57 plus GST
These prices for cash and delivery is mandatory, included free for purchases over $500.
Castrol tech guys said A55W30 is high quality. Excellent for cold starts - no need for any extensive warm up even in winter (I live near Canberra). When I described my driving - 35,000 km per annum 80% + on highway and expressway (little stop / start) he suggested I did not need to change oil each 7,500 km as the high quality of the oil coupled with my driving profile meant there was no need - would not hurt but unnecessary expense.
So, who knows how much oil goes into a GT?