On one hand I agree with all above comments. On the other hand there are a couple other factors to consider I think. First, I believe the main advantage of synthetic oil is not in extending change intervals but in gaining the technical advantages it provides, specifically in the areas of resistance to thermal breakdown at high temp, and superior flowability at low temps. Tom W makes a good point about not using synthetic if the car uses oil but he doesn't say why. One reason is that synthetic resists oxidizing so much that it will not burn completely. So it goes through the engine unburned and will clog up your O2 sensor. I discovered this in my Toyota the hard way. What drove this home was when I spilled some oil on my gravel driveway. I got my propane torch to burn it off. At first it burned a bit, but there was a portion of it left behind that would not burn no matter how long I left the flame on it.

And there are other issues I learned about when I did a used oil analysis at Blackstone Labs. That is interesting to do at least once if you are interested in such things which it seems you are. So in conclusion, I would (and do) feel much better having high quality oil for 6K or even 10K miles, over lower quality oil for even 3K miles.

Now let us never speak of oil again...

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra