I have a decent pile of old white papers that were published by the AES; you can access them cheapest by paying a one time fee for a year of access or pay a tariff per download. Among the things I found were various papers on the subject of wear, static and the like, along with various patents that are part of the public record (you need no AES access to obtain these) addressing surface noise, and again, static. I found very little to no discussion of "mold release" compound or agent as such. One paper, by S.K. Khanna, from RCA circa 1977 did address vinyl compounds in some depth, including the chemistry of PVC, polymerization processes, general characteristics of PVC resin as used for records, and then contained a discussion of various formulation variables, including the resins themselves, stabilizers (for heat and lubrication during compounding), colorant (which the author noted was used to "hide" plate out problems); fillers to change the visco-elastic properties of the compound and to reduce noise. Khanna also made note of certain "special additives" including lubricants, modifiers, plasticizers and anti-static agents, all of which have an impact on "flow". The author observed that the compounding process was complex, more "art than science' and at that time- during the height of vinyl as a medium, urged that basic research needed to be conducted into materials science to address the needs of "quality-conscious" persons. What this tells me is that even during the "golden age" the medium didn't follow one practice.
A couple other things to note: remember the oil crisis? It led to a lot of shoddy vinyl. I don't think that's when recycling started, but it probably become more widespread; Albert P noted in another thread that he has found debris embedded into vinyl; i have, as I'm sure others have too. One last thought- again speculation on my part. If the mix is made up of different (recycled) materials, each with their own chemical and heat/flow/etc characteristics, this could make manufacture far more problematic. I don't know, but I suspect one advantage of "virgin" vinyl, isn't that it is "pure" but it is probably more consistent. I offer this for what it is worth, not as an answer. As Miller said in Repo Man, I think about this stuff on the bus.