This is a really excellent thread. It gets to the heart of the debate (any debate) about oil. Is the difference worth it, if there even is one in the first place?
To me there are so many smaller factors that cloud the picture that when you consider them along with the big ones, the $ difference between conventional and synthetic disappears into uncertainty:
If you do fewer OCIs with synthetic, you should really factor in the value of your saved time. If you enjoy that time spent working on the car, turn that upside down.
How about the difference in resale value if you use synthetics. Whether or not they're better, it will give most buyers an impression of better maintenance and more remaining life.
If you intend to keep your car "forever," what are the odds that it will actually happen? Will it rust away before you get the chance? Will your wife/kid/self total it with a minor mishap when its value has become so low its not worth fixing? Will you change your mind and buy something else (most likely IMO)?
And what are the odds that, as some on the board believe (like me), conventionals are equal to or even better than synthetics for wear? One should discount one's calculations of expected benefit for the possibility that that is the case.
And how about the difference in fuel economy between oils... maybe 1% or so.
For me, with my E30 325i, a "forever" car, over say 200k miles, conventional on sale/cheap filter every 6k: $367
Or 200k miles, Redline every 12k plus "best" filter (but for my car it's the same filter!): $749
The difference is $382.
If I value my "free" time at $20 an hour the extra 17 oil changes with dino work out to $340, making it a wash. At $40 an hour the conventional would become much more expensive to use than the synthetic.
Or, if I decide to sell the car, maybe I could get a few hundred more for it if the fact that I used Redline catches somebody's eye.
Or the expense could be completely wasted if something happens to the car.
Then again, consider this for perspective: at 30mpg and $4/gallon I will spend $27,000 (!!!) on just fuel for it over those 200k miles. What's $382 compared to that!?