A follow up to my post, above:

Synthetic oil was, I believe, originally developed for jet/turbine engines. As such it had to have the ability to provide lubrication in hydrodynamic bearings in which shafts were spinning at mind-boggling speeds. It has to have the film strength necessary to protect the engine, but still low enough drag (viscosity) to allow the engine to start and operate. Further it had to resist thermal breakdown, because certain parts of a jet or turbine engine get very hot, and stay hot.

I have a Commercial Pilot license, and have several friends who hold both ATP (airline transport pilot) licenses, and A&P (Airframe & Powerplant) mechanic's licenses. They have all mentioned that you never change the oil in a turbine engine, except at overhaul time. If you need to change the oil in a turbine, that means that there is something severely wrong there. Edit to include: Oil in a turbine engine should not get dirty.

Now, think about that. Also, think about the design of a turbine engine. The gas flow in them is ducted and controlled every inch of the way. They have much less opportunity for the oil to get contaminated than a piston engine does. Synthetic oil, originally for turbines, was designed for a different purpose than the oils developed for use in cars. In a piston engine, the contaminants that enter the oil (around the piston rings, valve seals, etc.) are of paramount concern. These are filtered out by the oil filter and kept suspended by the additive package. Unless there is a dramatic difference in the additive packages in synthetic oil, I don't think they can "hold" any greater amount of crud in suspension than a dino oil can. And, as stated, our cars are old, and not turbine powered, I'd think the oil's ability to handle crud is the most important concern we face.

Note that on the bottles of oils for extended use, the marketing hype always mentions the improved or different additive package. Whereas the hype on the "regular" synthetics always mentions the resistance to thermal breakdown. Frankly I think the whole idea of selling synthetics for use in cars was originally driven by oil company marketing departments.

In these two posts I've been doing some thinking-out-loud, and am not claiming to be laying down the absolute truth, but just what seems reasonable. Intelligent dissent, backed up with facts, is always enlightening, and welcome.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)