RE's prices are pretty good.
I don't know what they use now, but back when they did my high flow, they put Martini Racing 10w40 mineral oil in and it drove like it was being strangled.
They were also using Castrol Edge (you have to ask for it), but I don't link how that compares in my car either.
As I said, if you are not picky about what fluids are used, go for it. Ray/Brad/Michael do good work.
Otherwise, you could always take your chosen fluids to RE for use in your service.