It seems that the spiced oil / reclaimed oil / bhaji oil has been done and done again, and it may add something and be, or has been, industry practice for whatever reason. Can it be undisputedly claimed it to be the mysterious missing taste? Probably not! This is because others dispute it, or have tried it and found it to be not really worth the effort. I've tried it and I agree it has to add something. One thing it will certainly add is a level of commonality to all dishes it's used in. If there is spiced oil to be reclaimed and used, I sometimes do it. However, my "best" curries have not come from it's inclusion. My best curries have usually come about accidentally, resultant of my habit of straying from the recipe instructions and / or ingredients slightly through laziness, clumsiness, forgetfulness, ingredient availability, experimentally and / or inebriation level. Unfortunately for me, it isn't from knowledge or skill and I rarely document everything I do and consequently my occasional accidental successes are unrepeatable.
While it may theoretically be possible to simulate used commercial oil at home (CT#1 recently called it manky oil *1), it is probably unrealistic. If this is a critical element, I would have to agree with Pete that commercial BIR base gravy is not achievable at home or at the very least, difficult. Alternatively, we would need to go and source some commercial BIR used oil and use it to make base gravy at home. The fabricated attempts at spiced oil must be very close.
I agree with Phil in as much as the scale of the base gravy itself is irrelevant, all else being constant. Prolonged cooking is possible and flavor development by aging in traditional curries and stews is widely known and accepted. However, I just used Latif's base gravy that's been in the fridge for nearly a week and really can't say it was any different.
For the most part, I'm OK with what I'm producing. It may not be top notch tucker but it'll do for now and it often does pass the cold test for me as well.
Either way, the information that is supposed to be "secret", actually isn't. It is on this site and others.
*1 Manky def: adjective 1) Inferior, worthless, 2) Dirty and unpleasant.