In both cold weather, and hot, you'll occasionally get some "foam" that accumulates in the oil fill cap. This is coming from experience with many different vehicles over 3 years as a quick oil change tech.
Combustion of fuel creates CO2 and H2O in perfect conditions. That H20 mostly goes out the exhaust in the form of steam, but some gets blown by the piston rings and into the oil sump. When the engine is hot enough, it's boiled and vented back through the intake via the CCV path. Some moisture and oil vapors float around until they find a cold enough spot inside of the engine, and then condense on the surface to form this oil snot that you find. Most often this is under the oil fill cap, or around the top of the dipstick. Some engines have long oil fill tubes, so they are cooled by air passing through the engine compartment - perfect spot for cool enough temps for condensation of this water in the oil.
If all you find is a bit of snot, and the oil at the end of the dipstick is fine, you'll be OK. This bit of oil foam is normal.