Answer 1. Many synthetics are made with synthetic base stock(s), using the same additive package that are used in the non-synthetic oil. As a result, while they are usually considerably better than the non-synthetic, they should probably not be used for extended drains without using oil analysis. On the other hand, any of them should be fine for the auto manufacturer's recommended drain interval. The only oils (that I know of) which carry recommended extended drain intervals are AMSOIL and Redline. Both of these are capable of doing as advertised. (You may consider this biased, since I've been using and selling AMSOIL for almost 25 years now--and using extended drains.)

Answer 2. You have to keep in mind that Volvo drain recommendations are based on European oil specs, which are considerably more stringent that the American API specs.


Synthetic is far superior to non-synthetic in many ways--better protection, less wear, cooler oil temperatures, less oxidation (self destruction), easier winter starts, and maybe even a bit better fuel economy.