Synthetic blends and semi-synthetic oils are essentially the same thing. Both refer to a motor oil that uses a combination of synthetic and conventional formulas. Because no regulations determine how much synthetic base oil must be used, the types can vary greatly.

Overall, the synthetic blend is going to cost more and last longer than conventional. Yet, it may not be as good as full synthetic, but it should cost less.

Pros

There are many benefits to choosing synthetic blends over conventional oil. Here are a couple of reasons to consider it. 

• Lower cost than synthetic: You can get many of the same benefits as synthetic oil but for a slightly lower price.
• Increased fuel economy: With more lubrication for the engine, it doesn’t have to work as hard, so you should see better fuel economy.
• Reduced emissions: The better the fuel economy is, the lower the emissions are.
• Special detergents/additives: Companies put in special additives and detergents to boost the engine’s well-being.
• Longer service life: You don’t need to change the oil as often as conventional.

Cons

There are some troubles with synthetic blends that are important to recognize. Here are the major complaints.

• Cost more than conventional: You have to spend more than you would for a conventional oil change. 
• Formulas differ: Because there are no regulations, you could be paying for something greater than conventional, but the formula may not be too much different. 
• Not as accessible: Not all shops use synthetic blends. In fact, there’s speculation that these could be removed as an option in the coming years due to a big push toward full synthetic.