The Castrol Professional OE oil can be purchased from the dealer, as can the filter (and gasket/seals). Not really sure what the significant difference in the gold-bottle Castrol and the "Pro OE" version is? Possibly viscosity only? Audi and others use the Castrol Pro OE gold bottle oil, so it's not exclusive to Jag. BMW's OEM oil used to be Castrol, but is now Pennzoil (Royal Dutch Shell), if that matters.
If the car has the belly pan that must be removed to access the under-side of the engine (to et at the drain plug and filter, as the last-gen GTO has), that will explain the higher labor charge (with many dealers being close to $140.00/hr labor rate). So, to me, the bulk of the oil change charge is the labor due to the car's construction features, NOT specifically the cost of the oil and filter.
So, shop the other import brands which use Castrol Pro OE branded motor oil. See if their viscosity matches what you need and how much they'll sell it to you, over the counter, for. For our used car dept, we normally buy the OE-spec motor oil (MB, Audi, BMW, etc.), which are all syn oil, for less than we can buy similar oil from the local auto supply chains. Like about $8.00/qt.
The OTHER thing is to go into the motor oil websites and see which non-Castrol oil brands will meet the Jag oil approval spec. It could well be that better oils are available, but only research can verify that. Plus look at
www.bobistheoilguy.com oil forums for "Virgin Oil Analysis" of Castrol Professional OE oils. Seems like the last stuff we got for a supercharged Jag was 0-20? It's my suspicion that all of the Castrol Pro OE oils are the same as the normal gold-bottle Castrol Edge oils, but I haven't proved that just yet. Research what that Jag oil spec means, too. Might be just the premium syn oil in the specified viscosity? To me, the premium syn orientation in the correct viscosity can be more important than the brand. Personally, I like Castrol oil, from my own experiences, but things have changed since then (1980s).
To me, a low-to-the-ground car with a top-side-accessible oil filter, would be a prime situation for a vacuum extractor oil remover. Just have a somewhat calibrated container the extracted oil would be deposited in, compared to the dip stick reading just before to the extraction, so you know you got most/all of it out.
Enjoy the Leaper!
CBODY67