This seems to have gone on two paths. Present and historical. Secret Santa at first made a comment about the curry sauce being "his indicator" of being "there". This is a reasonable post in line with the thread topic, and I'm assuming that he meant now, and not pre-90's. What was available 3 or 4 decades ago is past. Talk about what we can and / or can't and will and / or wont do today.
At the top of Page 2 (reply #10), Pete quoted SS and made the comment about the secret to the sauce being the base gravy, which is not on this or any site, ie, now. Hence my question to him about the "secret". Call it whatever you like, the Secret, the 5%, the last piece of the puzzle, that missing something, whatever. I thought Pete's post was suggesting he knew what it was and we all didn't so I pressed on it. That's how I read it but it may not have been what he meant.
If it has now simply come down to deep fryer oil, then I'd suggest that this information is on this site and others, contrary to Pete's original claim.
When my family ran a Fish n chip shop decades ago, once the fryer oil had reached replacement time, the oldest oil was gone. New fresh oil went into the chip pre-fryer and all the other oil was moved along. The chip oil went into the fish pre-fryer, the pre-fryer oil became the finish fry oil and the expired finish fry oil was gone. I understand that fishy infused oil becomes rank (manky) and is a different thing to used Indian spiced fry oil but I'm not sure I'd bother to use it. I guess if I could obtain some I'd give it a try. I know a guy who cheffed IR. I'll ask him what he knows about oil reuse.
If reclaimed oil is all Pete was referring to, it's been done. It is on this site an elsewhere. If not,???