Oil would float at the top of the water, so it would calm waves indeed. It's not a surfactant, in fact, it increases surface tension.
In a rig, there is no surface exposed to air, and no oil to float on top. Depending on the oil, it might attach itself to tubing walls/copper channels, and you definitely don't want that, unless you're running an oil cooler.
Liquid laundry detergent is something that I proposed some time back. It contains a surfactant, but is a bubbless soap, so pretty much ideal for watercooling. Dishwashing soap (the machine dishwasher) would work too. I would definitely stay away from the powder formats, because I don't care to try to dissolve this stuff in water, so that there is no powder residue to damage the pump. If it's already liquid, then it's not a worry (but I'd still feel it between my fingers for any kind of granularity).
Of course, true to my avatar name, I haven't tried it, so if you do, you're on your own, but we would all like to see results.
