saponification
You are right about oil floating. I should think, however, that either being "caught" in the detergent/soaps molecules or having, itself, become soap through the saponification process, that it would flush up and out with the water and not lie as scum on the clothes.
At least this has been my observation when spin drying clothes that had been truly filthy dirty.
I do believe there were pictures up on our forum at one time or other showing the difference in visible residue between neutral drains, solid tubs and hole-y tubs, were there not?
Jon, I do know Persil. It is one of the most effective detergents for dirty clothes I have ever used.
Shout gives me a very bad headache.
I confess to using WD-40 (outside) liberally on bad fat and oil-based stains. Seems easier to remove its residues than fight the heavier oils. Got a lot of transmission oil on my favorite jeans last year taking a Frigidaire TL (not a real one, sh** by White) apart. Couldn't use anything but what was under the kitchen sink. The enzyme based automatic dishwasher detergent and two trips through the Frigimore FL did the trick. Would not do that with this machine too often - too much aluminum exposed (spider).
Hmm, anybody have a machine which would stand up to a strong lye solution?