Given the background description of some cooking related query, summarize the question into a title

I am relatively new to deep-frying. Having seen the questions about oil reuse and conservation, I'm still at a loss about what's going on with my oil...My sunflower oil foams like crazy. I've been reusing this oil for a while now and was going to discard it, however a friend of mine told me this oil was still looking good. Relatively clean (I filter it regularly), no off smell and it doesn't smoke (in my fryer, that is). I only fry potatoes in this batch of oil to avoid flavor transfer. These are cut into French fries, chips or straw potatoes. Specially the chips and straws retain a lot of water, and I guess that has something to do with it.Also, I noticed the frying takes longer with the foaming oil.Edit: I've tried poaching(?) the French fries, but they foam during poaching. Then when frying, they really foam. I have to keep an eye on it or the oil spills over the top. Anything I can do about it, or should I just go ahead and discard the oil altogether?Edit: Serious Eats has an article about frying oil.
Frying - Oil foams

Most recipes for sponge cake use a quantity of yogurt which sometimes is replaced with milk, or even soja or almond drinks. In those recipes (flour, sugar, eggs, butter/oil, yogurt/milk, some flavouring), what is the role of the dairy product? Is it about protein/fat/chs, about water, about flavour, about colour? Do the substitution for soja or almond drink make sense, or exist better ones?
What's the role of milk or yogurt in sponge cake?

When the heat of a charcoal grill gets too hot, what is the best way to douse the fire; the objective being to lower the temperature without putting the fire out completely?How much water should be used when burning charcoal in a very primitive grill or pit without a lid or adjustable vents, and how should it be added (poured, sprayed, etc.)?
What is the best way to douse a charcoal grill?