If my memory serves me right. In general the owners manuals states change the oil every 7,000 miles (or so) or 6 months which ever occurs first for normal driving conditions. It wasn't until your quick oil changers started to pop up (Jiffy Lube & others) that they started promoting the 3,000 mile oil changes. I wonder who benefits from that.
From what I have been told that the oil itself doesn't wear out, but the additives in the oil wears out. That's why a 6 months intervals are recommended regardless of the miles because the deposits in the oil will eat away at the additives in the oil. That's why when changing the oil it is better "Change oil hot" to flush the deposits.
One concern I would have using synthetic oil. Would be having the same oil filter on the car for all those miles. I would think one would want to change the filter at least once in between oil changes.
I'm a boater who changes the oil at the end of season when I winterize the boat (around Nov). I also change the oil when I commission the boat in the spring (May). Because of the deposits that were in the engine that got flushed into the pan (eating away at the additives in the new oil for six months) when the oil was changed in the fall. The reason I change the oil in the fall also is so the deposits are not sitting there for six months eating away at the bearings and other internal engine parts. But I'm sure not to many boat dealers/marine mechanics would agree with this.
I also had a mechanic that once told me you cannot change the oil enough. Meaning you can only do good by doing so. I'm sure when you change the oil you can feel/hear the difference in the motor.
By the way I just discovered this site and it's great. I have 1973 280 that I'm getting ready to try to restore and I'll have plenty of questions.