A great deal is written on oil change intervals and from many perspectives. Each appears to slant the facts about oil changes to fit their position and it is difficult to know what is best.
Vehicle manufacturers often recommend 7,000 mile plus oil changes. Some synthetic oil makers claim oil changes up to 20,000 miles. A few self-proclaimed experts state changing oil is not necessary at all. Each has reasons for their position and none may be in line with the best interest of the vehicle owner.
The arguments in favor of extended oil changes are largely based on oil breakdown. breakdown is the point at which oil no longer functions properly as a lubricant. In use, breakdown is far from the only consideration. Several things happen to oil as it is used in an engine. Obviously it gets dirty. This is why oil filters were made standard equipment in the 1950's.
Motor oil today is NOT just oil. 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 motor oil what it is. Through use, oil additives may be depleted. When this occurs the oil no longer performs as it was designed.
Ethylene or propylene glycol, from coolant, can also find its way into engine oil along with combustion byproducts . Moisture forms, due to the heating and cooling cycles of the engine and is also trapped by engine oil. These liquid contaminants are not removed by the oil filter. Instead, they depend on oil changes, engine temperature and the PCV system to help remove them. Glycol is far less effected by temperature and destroys the ability of oil to protect the engine. Only a drain and refill 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 cheap, by comparison, this only makes sense. For many years the 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.
Miles are NOT a reliable indicator for oil changes
Using vehicle mileage is not an accurate indicator of oil change need. Three-thousand miles, driven continuously and at highway speed is very easy on oil. The engine is at full temperature and the PCV system is working. 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 or 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. This is why the number of miles driven does not determine the need for an oil change. The conditions under which we drive the vehicle 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.
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.
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. The truth is, synthetic oil gets as dirty or dirtier than conventional oil. Unless the engine is designed for synthetic oil, using conventional oil and changing more frequently may make far more sense.
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.
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.