I hear a lot of you asking the same question regarding this topic.
"What's the difference?"
Well, the difference is in refinement.
Conventional Motor Oils are refined crude oil with additives and stabilizers to help clean and protect your engine. Conventional Oils (mineral oils) continue to be used due to the cost of manufacture, and the relative durability of the oil. But at this level of refinement, there are still some impurities in the oil.
These impurities make the molecular structure of the oil uneven, which was fine until tight tolerances and more precise manufacturing methods evolved. The impurities really don't belong in your engine and will burn off, or leave deposits inside your engine that, over time can build up and cause failures. This is why it is VERY important to change your Conventional oil regularly and at a low mileage interval.
Synthetic Motor Oils are refined to the molecular level giving synthetics a much greater flow capability. The refining process for synthetics also greatly reduces the number of impurities in the oil, making it a cleaner oil to begin with. Additives to protect, clean, and extend the life of your engine are added as a bonus.
Smaller molecules flow better, faster, and more stable. Fewer impurities leave fewer deposits, fewer deposits mean less clogging resulting in longer engine life. Some additives are synthesized oils aimed to prolong engine life. Other additives include cleaning agents to help remove any deposits that may exist and agents to condition the seals and o-rings inside the engine.
Synthetic Blend Oils are just that, a blend of Synthetic Oil and Conventional Oil with additives for cleaning, protecting, and extending the life of the engine. Blends Must have less than 50% Conventional Motor Oil and more than 50% Synthetic Oil to be considered a Synthetic Blend. Consider this the "better" option of the three.
What we've learned:
Synthetic Oils are superior overall, with a longer oil-change interval. ~7,500 miles
Synthetic Blend Oils offer a good balance between quality and cost with a balanced oil change interval. ~4,500 miles
Conventional Oils will offer good protection and lubricity if changed at regular low mileage intervals. ~3,000 miles.