What's the word on Castrol Syntec? Good stuff, to be avoided at all costs (like FRAM), or about the same as your garden variety petroleum oil?

I've been running Syntec 10w30 in my '86 245 for the last 10K, and it seems to run a little cooler. Also have noticed a marginal increase in economy, from an average of about 25.5 to a little over 26mpg. I was using Castrol's "regular" 10w30 before. Tried the 15w40 Syntec in the summer, audibly changed the tone of the motor under load. Sounded like it was working harder to pump the heavier oil, or maybe I was just imagining things.

I know a guy who swears up and down that 20w50 is the best thing since sliced bread, but that seems like something one would put in a truck engine. Have read several times on the BB that Volvo does not mention 20-50 in the owner guide for a reason, i.e. don't use it. What application(s) is this oil intended for then?

TIA.

-Jim