The motor oil market today operates on three levels. Mineral oils are considered good; blending mineral and synthetic oils is better; and purely synthetic oils are deemed as the best. There isn't a standard definition of synthetic oil in the United States: Some synthetic oils actually use synthetic oils as the base, while others use a highly refined mineral oil as the base.
The best synthetic oils are making only small advances these days, while the lower rungs of the motor oil ladder are improving quickly. The biggest differences are in the additives in synthetic oils. As an example, speciality motor oil company Royal Purple has found a way to increase the film strength of synthetic oil, which protects where metal contacts metal inside the engine.
Some of the advantages of synthetics include:
- · Better gas mileage
- · Longer times between oil changes
- · Better cold-weather starts
- · Ability to clean out sludgy deposits in the engine
There aren't many disadvantages of using synthetic oils, but they do exist. Synthetic oils cost around 6 to 10 times the price of conventional motor oils. To make matters worse, synthetic oils will clean out the deposits that may be holding a weak seal together. This could lead to an engine oil leak that may cause myriad safety problems and cost you a lot of money to fix as well.
While we're at it, we should bust a few synthetic oil myths:
· You can't switch back to mineral oil after using a synthetic oil: You can switch as often as you like, with no harm done.
· Synthetics are too expensive: They also protect an engine better and need to be changed less often, which may make the expense of the oil worth those few extra dollars.
· Only high-performance and ultra-luxury cars need synthetics: Any car can use and benefit from synthetic oil's additives and longer time between oil changes.
· Synthetics damage seals: "A synthetic oil won't create a leak," said David Canitz, technical services manager at Royal Purple, "but it will find any marginal seals due to lack of maintenance."
source: howstuffworks, wiki