Look under your engine oil cap and you may see a milky, frothy residue. There are a couple of reasons for this — one being more serious than the other. In either case, it's not ideal for your engine.
At our auto repair shop, we typically see this due to condensation in the engine. This happens more often during weather changes and we also see it in cars where clients don't drive it a lot. Driving short trips doesn't allow the engine enough time to burn off the water vapours. Over time, this milky sludge builds up and accumulates. Your technician may suggest running a chemical through the engine to clean out this sludge, and you can help by driving the car for a longer duration.
For our clients who primarily drive short trips, we recommend taking the car out for a good drive where the engine can reach operating temperature and maintain that long enough to burn off the water vapours. Luckily for us in Markham (Ontario) there's a few good shopping malls within 30 minutes of highway driving so that gives us someplace to go! If your spouse has a longer commute, give him or her the car once every week or two so that they can take the car to work. Remember that the worst thing for a car is to have it sit for long periods, drive it periodically or use it for frequent short trips. Your car is designed to work and be driven!
Another (and more concerning) reason for the milky residue is because coolant has mixed in with the engine oil. This is a serious concern that could indicate a head gasket leak or engine damage. When engine oil is contaminated, this significantly reduces its ability to provide lubrication and may quickly lead to engine failure.
If you notice water droplets on the oil cap in addition to the milky residue, then most likely it's just condensation. If you have other symptoms including the need to top up coolant frequently, brownish colouring in the coolant reservoir, or white smoke coming out of the exhaust, then coolant may be mixing with the engine oil. You will definitely need to bring your car in for service before the problem gets worse!
Need help describing your car's noises? Check out “Does Your Car's Noise Sound Like A Raging River?” Want to know what helps your technician when you drop off your car? Here are “Some Essentials To Help Your Auto Technician.”