While the question about whether synthetic and conventional oil can be combined has been the topic of considerable debate, the fact is that they can. The truth is, both types of oil are derived as extracts of naturally occurring crude oil. The difference is that synthetic motor oil is derived from a higher (more refined) base stock, and goes through one of several synthesizing processes that leaves the molecules much more uniform in size. It also contains synthetic additives which increase the oil's film strength and ability to resist thermal breakdown.
While the base stocks for both conventional motor oil and synthetic motor oil are derived from naturally occurring crude oil, it may be helpful to examine the differences in how the two are processed. Both are distilled from crude oil, which passes through a sedimentation tower where mechanical contaminants such as water, sand and rock are removed. The crude oil is then heated to about 700 degrees Fahrenheit and pumped into fractional distillation towers.
After the oil is separated into its different components (natural gas, gasoline, kerosene, fuel oil, lubricants and tar) it passes through a series of extremely fine filters to remove any impurities. Finally, additives are mixed in to give the oil specific properties such as viscosity, higher flash point and better thermal breakdown resistance.
Synthetic oil takes the process even further in one of two ways. It takes the very small hydrocarbon molecules and combines them to make larger molecules with superior lubrication properties. At the same time, it takes very large molecules, breaks them apart, and then re-assembles them to produce smaller, more uniform molecules with a superior lubricant property. The synthetic motor oil also has additional additives which give it superior performance in terms of lubricating ability, resistance to thermal viscosity breakdown and antioxidant properties.
Mixing the oil types
Motor oil manufacturers sell conventional (regular) motor oil, synthetic motor oil, and synthetic blend motor oil which is a combination of regular and synthetic. As long as the weight and grade are identical, all three are compatible and pose no problems with respect to combining them. The one drawback is that when you add regular motor oil to synthetic motor oil, any impurities that may be in the regular motor oil get introduced into the synthetic motor oil.
Many people will create their own synthetic blend. Manufacturers do not make the ratio of regular to synthetic oil in their synthetic blends publicly available. The American Petroleum Institute, which sets minimum performance standards for lubricants, allows for the substitution of up to 25 percent synthetic into a conventional motor oil base without certification retesting.
Issues with mixing synthetic and conventional oil
Years ago, mixing conventional motor oil and synthetic motor oil was highly discouraged because it led to problems with gaskets and seals, causing oil leaks. As synthetic oils have evolved, this is no longer a problem with contemporary synthetic oil formulations. Since an insufficient amount of circulating oil in an engine can be catastrophic, leading to an engine seizing or throwing a piston rod, adding conventional oil to an engine that already has synthetic oil (or vice versa) is a perfectly acceptable option. Many motorists switch back and forth from regular motor oil in the summer to synthetic oil in the winter due to the superior viscosity maintenance capability of synthetic motor oils. If a vehicle has synthetic motor oil in it, using regular oil to keep the engine filled to capacity in between oil changes is a cost-effective way to stretch your vehicle maintenance budget.