Why use synthetic or semi-synthetic motor oil?

Synthetic or semi-synthetic motor oil offers longer time periods between oil changes compared to using regular motor oil especially for drivers who might be prone to ignoring their periodic maintenance oil changes. 


Synthetic motor oil is thinner than regular motor oil but withstands high and low temperatures slightly better than regular motor oil and therefore recommended for high performance racing engines or for use in very extreme hot and/or very extreme cold climates. Synthetic oil takes slightly longer to breakdown before losing its lubricating properties which translates into longer time periods between oil changes but it still requires periodic oil changes. Full-synthetic oil is recommended for extreme cold climate start-ups because its thinness makes it less sticky and less tacky between the moving metal parts of a cold engine which makes it easier for cold engines to turn-over compared to using regular motor oil which is a thicker oil but then it's the same reason why 5W(Winter)/40 or higher multi-weight motor oil is recommended for extreme temperature climates rather than choosing 10W/30. But synthetic oil is warned against using during a new engine's break-in period and also warned against using in engines prone to leakage because the thinness of the synthetic oil increases leakage particularly in older engines. 

Semi-synthetic motor oil is said to offer many same exact advantages as full-synthetic but at around half the price or so. 

Regular motor oil with an additive may be cheaper but it apparently works just as good as the more expensive synthetic oils and not worse nor better but essentially the same in regards to lubricating and protecting engines. 


sources:
wiki