What differences between synthetic and dino oil?

Wikipedia has a great page on Synthetic Oils, specifically their performance.

Pros:

• Better high and low temperature performance. They act more like a thinner oil at lower temperatures and like a thicker oil at higher temperatures, without the disadvantages of multiviscosity oils like 10W40. Especially useful when initially starting.
• Reduced problems with sludge, breaking-down, particularly in very high temperatures like in the turbo.
• Better lubrication properties.
• Extended change intervals.

Cons:

• Higher cost.
• Too slippery for properly seating parts during initial break-in.

It's not a case of one being unconditionally better than the other, for automobile applications. For example:

• Rotary engines such as in the Mazda RX series of vehicles should not run synthetic oil.
• Some vehicles come from the factory with synthetic oils, recommend using traditional "dino oil" for the initial break-in.
• There is debate about whether using synthetic oil in a car that has run regular oil for a long time is a good thing. The theory is that synthetic oils can clean out sludge and varnish that has built up. This sludge can dry up seals, and then when cleaned out leaks can develop in an engine that otherwise would have been tight.
• While synthetic oils are better in many regards, it may be that the average automotive applications aren't demanding enough to take advantage of them.

Although many people do not drive there cars hard enough to need synthetic oils, being able to go 10k miles before a change makes them cheaper in the long run, though more expensive per oil change. And when living in an area where temperatures get below -20, it can really help with starting. Also if you care about the environment, they are better for it too.

Oil still gets dirty. Syn might not break down as fast as dino and have better cold start properties etc, But engines still wear and there is still by-product of combustion that gets transferred to the oil. I would suggest changing the oil filter at 3000-3500 miles on a 7000 mile oil change with syn. On this annual change syn,same filter change interval ,3000-3500 miles. Sludge doesn't appear to be a problem with syn and it appears to offer better protection. But it's nice to have a choice.

I restored a 66 Chevelle SS with a 502 crate motor. Ran synthetic for two years burning a quart every 800 miles. Oil was black when I changed it. Ran dino for 6,000 miles at the advice of a mechanic. Oil after second change was amber and no burn. Went back to synthetic and no oil burn. Just my experience and don't mean to create a fight here. Just my experience.