Is Comma not tied in with Castrol now, I'm sure that I was on their site once and there was a link to Castrol products. I used Comma exclusively for my wife's old Fiesta 1.6Si with the Zetec engine, mainly because Motorcraft had not caught up with the needs of newer engines so at the beginning of her having that car, they did not offer an oil to cover that engine (Motorcraft being Ford's equivalent of VW Group's Quantum) - and Comma had!
Comma being OM supplier to some car brands, hum, not so sure about that - it has been hinted at by them for sales reasons.
I'd back the Quantum at a good price to your doorstep via ebay option.
Edit:- sorry maybe the "some car brands" was just me misinterpreting "some aftermarket only oil brands".