Just figured someone other than me may find this helpful?
I'm looking to set the record straight on that old chestnut: synthetic oil. It's a confusing topic, and there's a lot of rhetoric, largely because some manufacturers and peddlers of synthetics have made a lot of inaccurate and self-serving claims over the years. Some, actually a lot, of this rhetoric is pretty strident and opinionated.
Briefly, there are two types of "synthetic" oils on the market. Group IV oils consist of molecules that are synthesized from simpler chemical compounds. This lets the chemical engineers "tune" the characteristics of a lubricant to exact specifications. These oils flow more freely at extreme low temperatures and don't break down at very high temperatures. As a side benefit, they generally can be specified one or two grades lighter than a mineral oil, which consumes less energy as friction inside the engine and saves fuel. These are superior products, and command a premium price. Mobil 1 is a good example of a high-end Group IV. Group III lubricants are made from reprocessed petroleum products normally left over after making crude oil into gasoline, diesel fuel, heating oil and other products. They're more modestly priced, and have many of the desirable characteristics of the higher-priced spread. In much of the world outside the USA, Group III-based lubricants are not permitted to be marketed as "synthetic." Castrol Syntec is a good example of this type.
Want to use a synthetic--but experiencing sticker shock? You have two options: Hunt for sales and buy a case at a time, or use one of the synthetic "blends." With a substantial proportion of mineral oil in the mix, these are priced more affordably. They also provide only a proportion of the desirable characteristics of full synthetics.
Conventional mineral oils are simply one of the fractional distillation products from a barrel of crude, occasionally cracked from more viscous products in the refining process.
So, should you use synthetic oil? Depends. Some high-performance and high-end cars come factory filled with synthetic, and you should stick with it. If you live where it gets really frigid in the winter, you might be better off with a synthetic for its superior cold-weather starting. If you tow a trailer and your oil temperature is consistently above 200 F, you should use a synthetic oil and install an auxiliary oil cooler. I use mineral oils in my wife's car, but the race car and the motorcycles use synthetics. It's even more complicated than that--the race car gets a race-specific lubricant, with an additive package not intended for more than a few hundred miles between changes and not for very many cold starts. My Ducati, with its carbon-fiber-based dry clutch, uses an automotive-grade synthetic, while my wet-clutch Triumph uses a motorcycle-specific oil without friction modifiers. Wet-clutch bikes share the engine oil sump with the gearbox and clutch, so the friction modifiers used in auto-type oils might make the organic materials on the clutch plates too slippery and prevent good clutch lockup. If your driving cycle or your vehicle is more average, you probably can drive your car well past 200,000 miles without needing major engine work by using the proper grade of conventional mineral oil and appropriate change intervals.
But don't assume that if a synthetic is so good (read: very expensive) that you don't need to change it as often. The base lubricant may well be better, but the additive package, which can be as much as 25 percent of the volume of product in the bottle, can still become exhausted. And unburned fuel, partially burned hydrocarbons, atmospheric dirt, metal wear particles and blowby carbon particles will build up just as fast in a synthetic-lubricated engine as in one laved in petroleum-based oil. The only way to remove this stuff is to drain and replace the oil. I've always recommended 3000-mile oil changes, but I'm rethinking that. The air cleaners, the compression and oil-control rings and positive crankcase ventilation (PCV) in modern engines are better than ever. The air cleaners admit less abrasive atmospheric dirt, and closely fitted rings keep blowby and particulates above the piston and out of the oil. Sophisticated PCV systems are better at purging water vapor and partial hydrocarbons from the crankcase and burning them off in the engine, so I'm leaning more toward 5000 miles for most people with cars newer than about 1990 or so. As always, your mileage may vary.