i should have updated this post with the latest info. i forgot it existed, sorry guys.
another solvent was suggested to me that was much cheaper so i bought enough of it that i was willing to do a full scale test. i suspected there was residue left from the cleaning process i used, so i thought if i used more naphtha for the test then this should help move some of that residue elsewhere, rather than just settling back where it started.
it seemed to help, so i did another test with different sized pools of naphtha and the bigger pools definitely left less residue, with the biggest leaving nothing at all. i then repeated the tests with the original solvents and got similar results with the others.
a while after i discussed it with downwardsfromzero in another thread where he confirmed my suspicions. as he said above, these cleaning agents can leave their own residue. i was using washing up liquid, which i imagine is far from clean.
i plan to get some microscope slides for this purpose in the future. they will certainly be cleaner than my attempts. in the long run i plan to acquire all the necessary knowledge and equipment to do this properly, but for now they should be okay.
for the record, the clipper fluid was in every shop i tried when i was looking for newport. it seems they have dominated the market, which was very disappointing a the time. i traveled all across my city for hours looking for a shop that sold newport, or even something that was not clipper. after my improved tests it seems that clipper was the cleanest of all the corner shop lighter fluid options (and the cheapest), so i was hunting for what was already in my cupboard at home

although clipper fluid is the cheapest out of those it is still far more expensive than other options that are not branded as lighter fluid, so i am glad i went through that now as the solvent i landed with is much better bang for my buck