Engine oils have additives called "emulsifiers" which absorbe the blow-by moisture of combustion and "rust inhibitors" which tie up the sulfuric acid produced from the small amout of sulfur present in gasoline. The "emulsifiers" become exhausted if the engine is only driven on short trips (especially in cold weather) which prevent the oil from coming up to operating temperature and boiling off the water thereby freeing the emulsifier to re-absorb more water (NOTE: This is considered SEVERE SERVICE). The free water combines with the sulfur to form sulfuric acid which both forms rust and eats away at the internals of the engine. This is not a good thing!

Solution is to either drive the vehicle on the highway for 30 minutes once a week (to boil off the water) or change the engine oil more often.