The Big Check
Routine inspection before you start the engine should include checking the dipstick for the correct quantity of lube oil. If it registers too low or too high, there's a problem. Low levels could be caused by leaking oil or oil being burned by your engine. Some loss may be normal if the rate you're losing oil is consistent; a pint every 50 hours, for example, can be expected of engines that are long in the tooth. A faster rate can be indicative of more serious problems. Oil that appears higher on the dipstick (unless added by the operator) can be caused by seawater or coolant leaks into the crankcase. Common sources of these leaks are the oil cooler, exhaust system, or head gasket. Raw water mixed with lube oil will resemble mayonnaise. Coolant and oil will look more brownish, like old varnish. However, if allowed to sit undisturbed long enough, the water and oil will separate, with the oil on top. If this is the case, you can confirm this by draining a sample of oil from the bottom of the pan, from the drain hose, if equipped, or by inserting a small tube into the dipstick fitting. If water comes out before oil, pressure testing and further diagnosis by a mechanic will be required to trace the source. If you suspect water in the engine oil, it's better not to start the engine, which will only redistribute this mess through the bearings and oil galleries.