Thats very interesting JAH. That would indeed probably have had an impact as you say.
My experience was with a companys figures made in a rather soft, rubbery sort of plastic. At the time I used enamel paint, & they started to crumble barely a year after I painted them!! These figures were "modern day" so not involved in the 70s issues you mention. Whether or not something similar involving the oil content was at fault, I have no idea.
However, since painting some more of these same sets with the rubbery plastic, this time using acrylics, I have had no problems......not yet anyway!!!!