Tuesday, July 23, 2024 Detailed Auto Topics
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A great deal is written when it is best to change oil and from many perspectives. Much of what is written appears to slant the facts about oil changes to fit their position and it is difficult to know what is best.

The manufacturers of many vehicles recommend 7,000 mile or longer intervals between oil changes. Some synthetic oil makers claim oil changes may be extended up to 20,000 miles or more. A few self-proclaimed experts even state it may not be necessary to change oil at all. Each position has reasons yet none may be in line with the best interest of the vehicle owner.

extended oil change intervals may not be in the vehicle owner's best interest

The arguments that I have seen in favor of the extended oil changes are largely based on time to oil breakdown.  Engine oil breakdown being the point at which the oil no longer functions properly as a lubricant.  Far before the point of breakdown is reached, problems may be occurring.  Breakdown is far from the only consideration.  Several things occur as the oil is used in an engine.  One function of engine oil is to clean the engine.  This is why oil gets dirty with use.  It is also why oil filters were made standard equipment in the 1950's.  The oil filter helps to remove particulate matter above a certain size from the oil.

Motor oil today is NOT just for lubrication. Motor oil is a complex mixture of lubricants and additives. Viscosity modifiers, seal conditioners, detergents, anti-foam agents, corrosion inhibitors and dispersants are a few of the additives in motor oil. Oil additives make modern motor oil what it is. As engine oil ages oil additives may be depleted. When this occurs the oil no longer performs as it was designed.

Glycol is far less effected by temperature and may not be drawn out.  Glycol can also destroy the ability of oil to protect the engine. Only a proper drain and refill of the crankcase removes glycol that leaks into engine oil.

Old oil does not protect an engine in the same way that fresh oil does

The point of the oil change is to replace the old oil and filter before any damage begins occurring. Since engines are very expensive and oil is relatively inexpensive this only makes sense. For many years the oil change recommendation was based on miles of use and 3,000 miles was the standard.  With 3,000 mile oil changes engines were well protected, in all driving conditions.  Seal conditioners helped keep seals pliable.  Detergents kept engines clean and sludge build up and corrosion were much less of a problem.  Oil consumption was also much less of a problem as the crankcase was likely to be refilled before it ran very low.

Miles alone are NOT a reliable indicator for oil changes

Using only vehicle mileage is not an accurate indicator of the need for an oil change. Three-thousand continuously driven miles at at highway speed is very easy on engine oil. The engine is at full temperature and the PCV system is working as designed. The engine is operating near peak efficiency and fuel use is minimal. This is one reason highway fuel mileage is much better than in-town fuel mileage.

Most vehicles are not operated at continuous highway speed or ideal conditions. Trips of three to ten miles in stop and go traffic are much more common. Engines operating under these conditions rarely get to full temperature and do not operate nearly as efficiently. Stop and go fuel mileage reflects this and is far lower than highway mileage in most cases.  This is why taken alone, the number of miles driven does not determine the need for an oil change.  The conditions under which we drive the vehicle more accurately determines the need for an oil change.

The type of driving determines oil change need more than the miles driven

Vehicle manufacturers consider short trips and stop and go driving as severe service. This is where much of the confusion arises. Most people may consider the way they drive as normal, not severe.

typical traffic equals severe service 

Manufacturers want their vehicles to appear to be low maintenance. Recommending 7,000 mile oil changes, along with the statement under normal conditions accomplishes this. The point is most people do not operate under what manufacturers define as normal conditions.  Several short trips of less than ten miles at a time does not allow the engine to reach full operating temperature.

Synthetic oil producers wish to make their products appear cost effective. It is difficult to justify paying three times the cost for a product that simply adds better protection. Synthetic oil gets as dirty or dirtier than conventional oil.  It is also as prone to liquid contaminants as conventional oil.  Synthetic oil offers more protection, but is not a reason to extend oil change intervals.  Unless the engine is designed for synthetic oil more frequent oil changes using conventional oil may make far more sense.  When synthetic oil is specified it should be used.

Changing oil based on things other than mileage

Some operators of fleet vehicles are starting to base oil changes on fuel usage, rather than mileage. For instance replacing oil for every 200 gallons of fuel burned. This may be closer to reality than looking at miles. A vehicle getting 30 miles to the gallon with all highway use would have the oil changed at 6,000 miles [200*30=6,000]. A normal combination of highway and stop and go driving might reduce mileage to 22 miles per gallon. Here the oil would be changed at 4,400 miles [200*22=4,400].  All in-town use, in stop and go might give 15 miles per gallon and result in a 3,000 mile change [200*15=3,000].

Arriving at the exact number of gallons of fuel that indicates the need for change may take a bit of experience, but the method seems promising. Time must also be considered with vehicles that do not see much use. For instance, some vehicles may see less than 5,000 miles in a year. Such use is considered severe and oil should be changed at least twice a year, regardless of mileage.

Considering an average 14,000 miles driven in a year, the cost of oil changes are very small, compared to potential engine repair. The cost of replacing an engine or even a rear main seal, will outweigh the cost savings, many times over. Changing our own oil is even less expensive and often results in a much higher quality job.

Not changing oil can cause seals to harden and create oil leaks 

Many repair shops have seen a drastic increase in major engine repair, such as timing chain failure, leaking gaskets and seals and oil consumption, since the adoption of extended oil changes. The old commercial comes to mind, with the mechanic stating, "pay me now or pay me later." The cost is definitely more expensive later, choose wisely.

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