Recipe for a "pourable" basic sponge cake batter?

I think you want what is called a Hot Milk Sponge cake which is pourable. Here's a good recipe, http://whatscookingamerica.net/Cake/HotMilkSpongeCake.htm

I'm three for three on wondering what your goal is. What we think of as a "sponge cake" is actually a chiffon cake, which is a separated egg sponge. The yolks are whisked together with a portion of the sugar, oil (always oil, which gives it its lovely flexibility), some water, and usually vanilla extract. The dry ingredients (cake flour, leavening, and salt) are sifted together and blended in. The egg whites are beaten with some cream of tartar, and when very frothy, the remaining sugar is added veeeeeeeeeery slowly until the soft peak stage. Then they are folded, in thirds, into the egg yolk mixture. The batter is panned up, and baked immediately in deference to the egg whites, which contribute significantly to the cake's rise.

It isn't really a pourable batter. It really would help us to know what project you have in mind.

I'm curious about what you're trying to achieve with a runny sponge cake batter also. One of the things about sponge cake, as I'm sure you know, is that it's light and fluffy due to aerating the eggs. I've made sponge cakes that call for you to beat the eggs whole (and I've found that you can't really use a hand mixer for this kind of sponge cake--you need a stand mixer to get whole eggs to fluff up nicely), and then I've tried sponge cake recipes where you separate the eggs and beat the components separately. Either way, the batter is not exactly stiff, but it's not runny either. In my experience, cakes that have a runny batter (I'm thinking in particular about my favorite ginger cake recipe--the batter is so runny you're tempted to think it won't set) tend to be on the moist and dense side, not the light and airy side.
Also, many sponge cake recipes do not call for butter at all, and the ones that do seem to have you melting the butter before adding it anyway, so using oil instead of butter is no guarantee of a runnier batter.
I would pose the question to one of the resident baking experts, though. Boulangere may be able to shed more light on this.

why do you want it to do that? Use oil instead of butter, that will make it runnier