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Oil changes.  Are they really necessary?

This article is a bit long, but we urge you to take the time to read it. It is very useful in understanding the importance of regular and frequent oil change intervals.

What makes crankcase oil turn dark and dirty?

What causes engine sludge deposits?

The popular idea is that when gasoline is burned in engines, it’s changed to harmless gases which blow out the exhaust pipe. Unfortunately this is not the case. For every 100 gallons of gasoline burned in an engine (every 1,200 to 2,000 miles in passenger cars), the following things are formed in addition to exhaust gases: 

 

• 90-120 gallons of water 
• 3 to 10 gallons of unburned gasoline 
• ½ to 2 pounds of soot 
• ¼ to 1 pound of resins and varnishes 
• 1 to 4 pounds of nitrogen and sulfur acids 
• 6 to 10 ounces of insoluble lead salts (if leaded gasoline is used) 
• 1 to 2 ounces of hydrochloric and hydrobromic acids

 

Notice the large amounts and the variety of things formed from the burning of only 100 gallons of gasoline, besides mere exhaust gases. When an engine is good and warm from steady running, these various combustion products are mostly blown out the exhaust, so they cause little or no trouble. But when an engine is cold, due to frequent stopping, standing and starting, the cool cylinder wall acts as condensers for these combustion products. The water, soots, resins, acids and lead salts condense and adhere to the oil films on the cold cylinder walls, and then work down past the pistons into the crankcase oil. 

 

OIL SHOULD BECOME DISCOLORED IN USE

Good Heavy Duty detergent type oils are designed to keep combustion chamber blowby products suspended in the oil. This assures effective removal of blowby contaminants from the engine at each oil drain. Accordingly, detergent oils should become discolored in use; this is just evidence that the oil is doing its job. Although the oil may appear dirty a short time after changing, it is keeping the engine interiors clean, thereby helping prolong engine life. Under stop and start driving typical of car and truck use in cities and towns, surprisingly large amounts of these combustion products often wash down into the crankcase oil. This results in dirty, discolored oil and in severe cases to heavy sludge deposits. 

 

OIL TURNED GRAY : Caused by accumulation of lead salts in the oil. The tetraethyl lead anti-knock compound used in "leaded" gasoline is converted to insoluble lead salts during combustion. These lead salts have a light gray color, so it is very common that crankcase oils will develop a deep gray color in quite short mileage. These lead salts are harmless to the oil, and to the engine, as long as they don’t accumulate in excessive amounts. 

OIL TURNED BLACK: Caused by accumulation of combustion soot in the oil. Just a few tenths of a percent of the soot in the oil is enough to turn it coal black in color. Small amounts of this soot contamination are entirely harmless, but when it builds up to over 1%, sludge deposits are very apt to form. Soot contamination of oil is a direct indication of poor combustion and excessively rich fuel-air mixtures. (See next section of Sludge Deposits.)  

ENGINE SLUDGE DEPOSITS

There are three major causes of engine sludge and varnish deposits: 

 

• Oxidation and breakdown of poor quality, unstable oil. That is, oil which is poorly refined and inadequately treated with antioxidation and stabilizing additives. Such oils may readily form heavy sludge and varnish deposits in engines in severe service. This is termed "oxidation sludge". 
• Contamination of the oil with excessive amounts of combustion products. This is termed "contamination sludge".
• Oils having inadequate detergency properties. One chief function of a good Heavy Duty detergent type oil is to keep insoluble contaminants in dispersion in the oil, so that the insoluble matter won’t emulsify and form sludge and varnish deposits. When the best of high detergency oils are overloaded with insoluble matter, separation of sludges is bound to occur. 

 

Most name brand oils produced today (i.e. Shell, Castrol, Valvoline, Pennzoil, etc.) are all highly refined and reinforced with stabilizing additives. They have outstanding stability and resistance to oxidation or break down. "Oxidation sludges or varnishes" are minimized even under the hardest engine service. If sludge or varnish deposits develop in your engine, and you are using a high quality name brand oil, it is likely they are contamination sludges originating from engine combustion chambers. 
 

HOW SLUDGE AND VARNISHDEPOSITS FORM 

It takes a good 15 minutes of steady running to get the cylinder walls of an engine good and warm, so that the combustion products condensing and washing down into the crankcase is minimized. When an engine is used in a lot of stop and start service, surprisingly large amounts of combustion products will condense on the cold cylinder walls and then work down into the crankcase. This is especially the case in cold weather.

 

Contamination of the crankcase oil with water, soot, lead salts, gummy resins, and acidic matter first cause the oil to become dark and dirty in color, but this is not actually harmful. However, when the total insoluble contaminants may suddenly begin to emulsify and then plaster out on the engine surfaces as sludge or varnish deposits.

 

Operating conditions, including the mechanical condition of the engine, are extremely important in engine deposition, frequently producing ranges of effects larger than those produced by fuel and oil combined. Maintenance of good combustion, proper cooling jacket and lubricating oil temperatures, and satisfactory crankcase ventilation, together with the use of oil filters, and avoidance of excessively long oil drain periods, will each contribute toward improved engine cleanliness.

 

There is general agreement among investigators that door-to-door delivery driving* is the most severe type of operating condition so far as sludge and varnish deposits are concerned. This type of driving includes many passenger cars, such as taxicabs, police cars, and privately owned cars which accumulate low monthly mileages. The low cooling jacket and oil temperatures, and the protracted periods of idle operation, which are characteristic of this service, frequently cause premature engine difficulties and possible failures. Lubricating oil becomes loaded with contaminants in a very short period of time. 

Authors note: Although many car makers allow oil change intervals up to 10,000 miles, it is my opinion that this is too long between changes.

 

If you use a high quality synthetic oil (Mobil 1, for example) and a high quality filter (Mobil 1) that filters to 2-3 microns on a single pass then you can get by with changing your oil every 7,000 miles or so.

 

If you are using standard high quality oil (Castrol, Valvoline, etc.) and a commonly available filter (Fram, Purolator, Wix, etc.) you should change your oil every 3,000 miles or 6 months, which ever occurs first.

 

I cannot stress this importance of regular oil change intervals! More than 80 percent of the vehicles I have inspected that had an engine failure have had some sludge or varnish present in the engine. Regular and frequent oil change intervals will prevent this buildup from occurring. 

Jim A. Stone, Jr.  CEO

CarInspector.US, Inc.