|>>|| No. 5686
OP, I've been thinking about this over the past few days, and I think the correct answer to your question is that you've misunderstood the problem.
The "oily film" that you feel with a shower gel is not necessarily the result of any oils or moisturisers in the gel.
Ordinary soap (sodium tallowate/sodium stearate etc.), is very very bad at dealing with water hardness. Soap draws the calcium out of the water and forms soap scum. That "squeaky clean" or dry feeling you have after using soap is the film of soap scum left behind on your skin.
Modern shower gels have chealating agents added (commonly some variety of EDTA), this keeps the hardness dissolved in water, which leaves your skin feeling slick after it's cleaned. Even in the "sensitive" gels such as the Sanex which don't have a chelating agent in, the main sufactant (SLS) is much less affected by water hardness than a traditional soap, so it still gives more of an oily feeling than soap.
Try this experiment: Buy a pack of borax substitute. Dissolve a few teaspoons of it in a bowl of water, and then wring your hands in it. You should notice that the water feels weirdly slippery. It's not making your skin oily, it's because it's stopping any of the minerals in the water from sticking to your skin.