If you check your car's oil and find that the oil on the dipstick looks whitish and milky, there's probably water in the oil. This can damage the engine.
If the car's cooling system had a coolant recovery system the little plastic container connected by a hose to the top of the radiator -- and the car's water level keeps dropping, and the oil level is rising, and the oil on the dipstick is milky, there's water leaking into the oil. The repair needed here is beyond the scope of the average car owner.
However, if the oil on the dipstick looks milky, yet the coolant level does not drop, you may be able to solve this one yourself. All engines have moisture up the crankcase, the bottom part of the engine where the engine oil lies. Ordinarily, the engine heats up and the moisture passes harmlessly out the tailpipe. But if a car is used strictly for short trips the engine may never get hot enough to get rid of the condensation. Here, the solution is to change oil and filter about every 2,000 miles -- or every three months whichever comes first.
A bad positive crankcase ventilation system also can cause the problem, even if the car is driven a lot. The PCV sucks fumes and moisture out of the crankcase and they leave through the tailpipe. But if the PCV isn't working, moisture can accumulate.
Another possibility is an inoperative thermostat.
If the thermostat and PCV system are working okay, and there's water in the oil (and no coolant loss) -- then change oil and filter more frequently.
Of course regular oil and filter changes are important even if you don't have water in the oil.