By nature, all four-stroke engines require ventilated crankcases. The reason is twofold. First, when an engine is started or run at low temperatures, condensation takes place and fuel vapors migrate into the crankcase. These contaminants, essentially water and raw gasoline, mix with the oil to form engine-damaging sludge. Fortunately, once the engine reaches operating temperature the water turns to steam and the fuel remnants percolate out of the oil. As long as the crankcase is vented, either by the atmosphere or by using a positive-pressure system, these nasty byproducts are free to drift out of the engine and go their merry way without causing any harm.