Engines are like the human body, exhibiting all sorts of strange symptoms that may indicate a severe problem, a minor one or nothing at all. The trick to telling the difference is to look past the obvious -- there's foam in the oil -- and try to determine the nature of the foam and where it happens to sit in your engine.
Short Trips and Cold Weather
Oil flows through dozens or hundreds of tight spaces inside your engine -- particularly in the valvetrain -- and every time it does it picks up a few air bubbles. Under normal conditions, these air bubbles would simply pop in the oil and release into the crankcase, but very cold oil tends to trap the bubbles and turn them into foam. It's the same thing that would happen if you shook a half-empty bottle of cold maple syrup; the surface tension of the highly viscous syrup would negate the bubble's natural tendency to rise and separate. Cold weather and making lots of short trips that don't get the oil up to temperature will almost always cause a bit of foaming.
Condensation in the Oil
Oil and water mix about as well as oil and air, which means that water trapped in your oil will produce a similar foaming effect. Generally speaking, water of any sort in the oil will emulsify to yield a dense, white foam, and air in the oil will make for a light, yellow foam. A certain amount of water in oil is normal, a result of condensation that builds up inside the engine. A loose oil filler cap will allow more moisture to penetrate your engine; the degree to which condensation will affect a given engine depends on where the condensation happens to pool in the cylinder head.
Condensation isn't the only place that water can come from, and it's not the most likely if you happen to find foamy oil on your dipstick instead of the bottom of the oil cap. A little bit of water in the cylinder head can cause foam because there isn't much oil there, but you'd need a lot more to emulsify the oil in your sump. If you see a thick, white foam on your dipstick, then you've got water from the cooling system pouring in through a leaking gasket. The intake manifold gasket and head gaskets are the most likely suspects, although the timing cover gasket may also be at fault if water must pass through it going to and from the water pump.
A light, yellow foam on your dipstick, particularly in cold weather, could indicate aeration in your oil pan -- typically a result of overfilling the engine with oil. Spinning crankshaft counterweights work as a very efficient blender, quickly whipping air and oil into a meringue-like foam. The solution here is to drain a bit of oil out of the engine. Oil may also tend to foam a bit more if your oil or oil additives contain large amounts of detergents, which are essentially soap. While less likely than a blown gasket, a cracked engine block or cylinder head will also dump water into your cooling system; in that case, a bit of foam is probably the least of your concerns.
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