As to the why, I don't have a good answer. I suspect it has to do with the way the engine wears and the metals get used to certain things. After so many miles the finer molecules of the full synthetic are allowed past the rings and the valve seals where the bulkier, less evenly sized blend or conventional will not fit. Similar to how an engine started on ether consistently will eventually become dependent and won't be able to start on its own anymore. I have read of mixed results switching back.
I would not switch to a conventional, as that isn't the specified oil either. I would use a "High mileage" semi-synthetic 5-20. The high mileage blends have additives that are specifically designed to help resist combustion, so they shouldn't burn off as easily.
As I said in my original post I consider the use of a full synthetic in a stock 3.0 to be a waste of money. For most engines full synthetic oils cost disproportionately more than the benefits available from using them. The only place they are really needed is in high-performance applications where fluid shear is an issue. It's something that is basically only needed if the manufacturer specifies it.
The difference between conventional, semi, and full synth is largely in how refined the product is, and the quality of the detergents used. The various "grades" are subjected to different levels of filtration at the molecular level to produce a more uniform molecule size as the grade (reads "price") increases. This helps reduce fluid shear under stress, which is a good thing, because shear will eventually break the oil down and it won't insulate the metal parts from each other anymore. This is where the idea that full synthetic lubricates better comes from. Its not completely false, but in the case of the 3.0 you just aren't making enough power to do that kind of damage to your oil.
The other factor is the amount and quality of the detergents added. Conventional oil has the least, obviously, and gets dirty quicker. Full synth has the most and will clean better longer, but for cleaning it still won't hold a candle to diesel oil, which is what I use when I can find it in the right viscosity. 5-30 diesel is hard to come by though. Detergent is important because most auto chem companies estimate that the oil filter really only works for the first 15-20 hours of operation, and after that it is up to the oil to keep things clean and the filter is just there to keep out large pieces of debris.
These two factors are what dictates oil change intervals more than anything. Run time, length of trips, engine temp, emissions controls, engine wear, and fuel quality all factor in as well, but breakdown from fluid shear and detergent quality/quantity are the two biggest factors. This is why synthetic oils usually have a longer OCI than the others. They have better stuff, and thus a better shelf life in the engine.
I personally just use the cheapest stuff I can buy, because I change my oil every 6 months regardless of miles driven. That's about the shelf life of most oils once the bottle is opened. Since I work about 3 miles from home and don't do much driving but to go to work and home again it takes me 6 months to rack up 500 miles. Especially now that I have my BII back on the road and am splitting my miles between two vehicles again.