How often to check your car's oil
by Douglas Samuelson - email:  dsamuelson@frii.com

Oil not only provides lubrication for the internal parts of your engine and transmission, it acts as a coolant and contains additives that help to keep the engine and transmission clean and to prevent seals and gaskets from becoming brittle. 

There is a misconception regarding the oil pressure warning light and/or oil pressure gauge Most people think that this light or gauge is an indicator of the amount of oil in the engine crankcase.  The light is designed to come on when engine oil pressure is lower than the normal operating pressure for that particular engine.  Oil pressure is the amount of pressure (measured in pounds per square inch) generated by the engine oil pump, to supply oil to the internal engine parts.  Granted, if the engine oil level is so low that the oil pump cannot suck oil from the oil pan, the light will come on, but by this time, severe engine damage may have already occurred.  The oil pressure gauge serves the same purpose, but it is a better indicator since it can detect a gradual change in oil pressure.  It is normal for engine oil pressure to decrease somewhat as an engine wears, but at no time should an engine be operated when the oil pressure is below the recommended operating pressure. 

Most engines can be operated without immediate damage when the oil level is one or even two quarts low, but they should not be run for an extended period of time.  When the oil level is low, momentary starvation of the oil pump can occur when the vehicle is in a turn which will allow the oil in the oil pan to "slosh" away from the oil pump pick-up screen.  This momentary lack of oil will not always be detected by the warning light or gauge. 

The best time to check engine oil level is when the engine is cold.  If the level is checked on a warm engine immediately after it is shut off, there can still be a small amount of oil in the upper engine that has not drained back into the oil pan.  Over-filling an engine with oil can sometimes be as damaging as a low level.  If the crankcase is too full, the oil can be "churned" by the rotating crankshaft counter-weights, causing the oil to be aerated. 

Some people will say "My car does not use oil.  Why should I check it so often?"  My answer to this is "S__t" happens".  An oil filter can leak if not properly tightened after an oil change.  An oil pressure sending unit can develop a leak.  A valve cover gasket or rear main seal can develop a leak.  The oil drain plug may not have been tightened by whoever changed the oil the last time, and in some cases the threads may have been stripped in the oil pan.

Most cars do not have an oil light or warning system for the automatic transmission.  Not unlike the engine, if the oil level is too low, damage will occur if the transmission "slips" or overheats.

The transmission oil level should be checked with the engine running and the transmission in the Park mode.  Most cars have a transmission oil dip-stick that will be marked with the proper level for "cold" or "hot" checking.  For the same reasons as the engine, the transmission should not be over-filled.  If you are one of the fortunate ones who own a car without a transmission oil dip-stick, your transmission fluid level will most likely have to be checked by a technician (brilliant engineering).

I have been involved in the automobile repair business for many years.  I have a habit of checking the engine oil on every vehicle that comes into my shop.  I do this not only as a convenience for my customers, but to protect myself.  If I did a tune-up on a vehicle, and the engine "blew up" on the way home because of a low engine oil level, the customer would more than likely, blame me for the failure.  You would be surprised to know the number of cars that I have found with little or no oil registering on the dip-stick. 

I do not mean to "single out" women, but I have found that a lot of my women customers are more concerned about windshield wiper blades and windshield washer fluid than their engine oil.  More than 50% of my customers are women, and in most cases they are more concerned & knowledgeable about vehicle maintenance than the men. 

Putting all "kidding" aside, check your engine and transmission oil every time you fill up with gas.  It could save you the cost of a new engine or a transmission rebuild.