Lets put it this way.
If you take any quality synthetic which to me is really only two (Amsoil & Redline) there are a few others close like the Rotella T6 and I think one of the many Mobil 1 variations like the European formula is actually 100% synthetic.
Take any of those, run 12,000 miles, get it tested by Blackstone.
Do the same with any Dino oil of your choosing.
What oil do you think will be in far better shape, higher TBN , additives left , less metals etc?
I can guarantee you that the difference will be substantial and that is just one oil change, do that over the life of the engine and without any doubt the one using the synthetic will have less wear if you change oil at over 10,000 miles.
If you change your Dino oil at 3000 miles you will be fine, but still doesn't change the fact that dino oil breaks down FAR sooner than does a quality synthetic.
Now for me personally I would NEVER in a million years let regular petroleum oil go past 3000 miles.
I only drive on average about 5000 miles per year give or take 500.
That would mean two oil changes with petroleum Vs one with any synthetic.
Thus I would save no money at all running Dino oil. Give or take a $1
The 25,000 or 15,000 "guarantee" is simply a marketing ploy, BUT it is a ploy based on easily provable fact.
Run ANY Dino oil to 15,000 miles and have it tested and see what you got.
Run any Dino oil in any car to 15,000 miles regularly and my bet is you will be hard pressed to get that engine to go past 150,000 miles before a rebuild.
That would mean 10 oil changes and you're done, LOL.
Do it with a synthetic and as long as you have a high quality oil filter it would be about the same as running Dino oil changes at 3000 miles in terms of engine wear.
So really the only question here is how often do you want to change your oil.
Cost wise there is little to no difference between Dino/Synthetic.
In fact if you do mostly highway miles and over 15,000 miles a year you will actually save money with Synthetic.
If you look at what oil does.
It coats the metal parts inside and allows them to run on a thin layer of lubrication thereby reducing friction and wear. It also works as an additional coolant, neutralizes acids, captures and transports combustion by-product particles to the oil filter, and employs solvents to keep the engine clean.
Now synthetic is slightly better at every one of these things even when both have 1 mile on them but after about 3000 miles Synthetic is considerably better at every one of these factors.
Another thing that no one has mentioned is Synthetic contain fewer contaminants like sulfur, wax, and other elements that contribute to sludge build-up. All petroleum based oils have these waxes and sulfur etc and after about 3000 miles once all those smaller particles have evaporated you are left with only large particles and those waxes turn to sludge.
Also think about the reason that it is never recommended to run Synthetic oil in any engine during the 500-1000 mile break in period?
Because the synthetic works so much better that it doesn't allow the engine to break in properly, the rings to seat, and that happens with the Dino from the first minute that you run the engine on a fresh oil change which is why the engine can "break in" in that 500-1000 miles or so.
So I think it is pretty clear that Synthetic protects the engine better no matter what, even if you change the Petroleum oil at 3000 miles it is still wearing the engine more than synthetic at 3000 miles.