i actually did the opposite thing and when i saw people doing Historical Lube Discourse arguing against the use of various utility oils for this purpose in historical fiction i was like. well why not???
if petroleum jelly and crisco are both notable modern improvised lubes from before the lube industry took off, and crisco was celebrated for being plant-based unlike vaseline and therefore healthier to shove up your ass, why would rapeseed or olive oils be A Huge Problem????
answer is, probably wouldn't. op has done a solid job laying out most reasons using A Random Oil is strongly advised against in a context where you can buy sex lube; disturbing the vaginal flora and more difficult laundry are up there, and Condom Issues top the list. and yeah modern 'random oils' are often petroleum products, which you want to be very careful about which ones you apply to your body and how.
list mostly does not include 'these don't work' or 'these are somehow hazardous in themselves.'
there is an element of lube industry propaganda circulating, i think, and a lot of people receiving and passing on advice about what is the ideal set of choices for them to make personally without any real interrogation of why, and then pouncing on deviations from this received norm even outside their own context. not that there aren't also some truly alarming improvised lube ideas out there lmao.
do want to advise that linseed oil is probably low on the list of plant-based utility oils to use for this purpose, even though it's not likely to harm you directly, for a weird reason!
it's notable for its volatility, in the sense of it has a lot of components that like to evaporate and that oxidize really dramatically. this makes it valuable for uses like paint and varnish, but also means that if you get careless with it, it can uh. spontaneously combust.
so if you make a mess of fabric with some linseed oil and then get distracted and it oxidizes and heats up and your discarded pants burst into flame, that's a lube problem you do not want to be having.
as a writer i desperately want an excuse to have someone's balled-up lube stained pants spontaneously combust under the bed, driving the plot forward, but like.
