Synthetic oil has a more constant viscosity than conventional oil does.
I build oil pressure faster on cold starts, the engine cranks over easier.
At high temps I carry more oil pressure with synthetics than I do conventional oils.
Also with a turbo, the thermal breakdown of the oil is much slower with synthetics, they can stand more heat.
The oil also carries more oil pressure after 2000 miles of use in my engine, I could always tell an oil change was getting close to due with conventional oil because the oil pressure would drop 5 pounds or so at highway speeds.
That is not true with synthetic oil, it stays the same.

That statement would be enough for some people to say I should extend the change interval.
But I want the soot and acids that are in the oil out of my engine.

I have been running synthetic oil for several years.
I also still change it at 3000 mile intervals.
The only downside is the cost, it was adding 1.5 cents per mile to my operating expense.
But my engine is fresh, I don't add any oil between oil changes.
My cost may increase slightly per mile, my drum of Mobil Delvac 1 is empty.
I am now running Mobil 1 which is a bit more expensive due to price increases.
Oil rating must be CD or CF which was the standard back in the 80's, CI-4 had not been implimented then.
CI-4 has more to do with engines like the Power Stroke which use high oil pressure to fire the injectors, that requires much more anti foam addative than the IDI engine does.
The difference between Mobil 1 and Mobil Delvac 1 was the CI 4 rating and the anti foam addative increased levels in the Delvac 1.

A couple other observations.
I tore an engine down that had 18,000 miles on it running synthetic oil, the cylinders looked like they were freshly honed.

I was looking at a zero turn commercial lawn mower, X Mark Laser.
The reservoir for the hydrostatic drive was only one quart in size.
I asked about the small reservoir and heat affecting the hydrostatic drive power since I am around hydraulics every day at work.
The answer was they use Mobil 1 engine oil for fluid in the hydrostatic drive.
The oil was still stable at 375 degrees with no loss in power.
Conventional oil would be smoking, thinner than water and turning black from heat alone at 375 degrees.

 

And if that doesnt convince you to switch to synthetic, then nothing will. Kind of like my grandpa, I tell him "Grandpa, you can type on a PC and print out your invoices for your business" and he replies "My typewriter does damn good, and I quite fine with it" LOL

 

OK, I am sold. One last thing though. If one usually ran for instance a quart of STP oil treatment per oil change, (I don't on my IDI) would there be an issue here? In other words, if you run synthetic, you would need to research any kind of oil additive, correct?

 

I run synthetic and like the benefits it gives me. I would not recommend additives the synthetic is already top notch stuff in my book. Mixing and matching I leave to the chemists. What I do like about synthetics in 2 stroke gas engines no plug fouling and when you pull them down they are like new inside. My oil pressure stays the same in my diesels and my backhoe which has no starting aids IE GP's or direct ether start. Spins quick and fires on her own even creeping up on zero.

 

No oil addatives for me either.
Straight synthetic oil and a Fleetguard LF3630 oil filter.