Which emulsifer to use for low viscosity emulsions of 10-20% fat and water
Milk 
You mention milk in your question, so let's start there. Milk is about 2-4% fat, 8-10% milk solids, and the rest water. It has your ideal viscosity, is great cold, and carries flavor well. Commercial milk doesn't separate &amp; is remarkably stable. (It's important to note that fresh-from-the-cow milk separates rather easily, and is therefore generally called &quot;cream top&quot; milk.)
It is pretty close to fitting your parameters. So how do they do it?
Homogenization
Homogenization is done by forcing milk though an extremely narrow opening at pressure above 2000 psi (and as much as several times higher). The combination of extreme pressure and turbulence causes the fat to break up into tiny bits.
The ultrasmall fat globules combine with the naturally occurring emulsifiers in the milk solids (whey proteins, casein) to form an emulsion that is a impressively stable.
Home Homogenization
You don't, really. I'm not aware of any home homogenization methods that would produce something as stable as milk, as you describe.
Why not? When I think of &quot;high pressure though a narrow opening,&quot; my mind goes directly to a pressure washer. Most of the home (especially electric-powered) pressure washers operate below 2000psi. Gasoline-powered and commercial pressure washers do get above 2000psi used at the low end of homogenization. Pressure washers are not rated for food safety.
Perhaps you could devise a way to ensure all the necessary parts of a gas powered pressure washer were food safe and sanitized, and spray your emulsion into a stainless steel tank for capture, and achieve some sort of very stable, low viscosity emulsion. But it doesn't seem practical, not to mention there being obvious safety concerns.
Another way?
Maybe? I focused on milk specifically because it's a concrete example. Your question is a bit more hypothetical, which makes a specific answer harder to nail down. Given a more concrete example, there could be simpler answer. Depending on what ingredients you want to emulsify, for how long you want to hold it, and how you want to use it, a reasonable combination of emulsifiers (like lecithin), thickeners (like starch, or agar agar), heat, pressure, mixing, and thinking differently might solve your problem. Whipped cream isn't a stable emulsion, but squirty whipped cream cans solve the problem a different way.