Haldi,
I finally got around to trying the "old oil". Mind you, mine was nowhere near black, but certainly was a medium brown colour. I must have gone through 50 sittings of chips over the last month to get it to this point.
Following your advice, I made a base with 50% old and 50% new oil. I used 2 tablespoons of "old" to make the Vindaloo.
Observations - firstly I was worried that the base and curry would smell weird, as the oil itself had a very "chippy" and 'chickeny" smell to it. I could not detect it in the base or the curry. I couldn't pick out a discernable "wow" taste - although I wouldn't know what a 1970's or 80's BIR curry would taste like in comparison to todays.
That said, tonights curry could well have been the best to ever come out of my kitchen. Its just so tough to rank one curry against another without doing a side-by-side. I was tempted to make parallel bases and Vindaloos to do such a test, but decided to do just the one.
Great work Haldi, I am going to continue to use this method, until such time as I get the chance to do the head-to-head.
-- Josh