My quest to separate fact from mythology continues. The question of the day is whether sythetic oil provides better fuel economy than conventional dinosaur juice. The answer may seem obvious, but there have been claims that there's a benefit. A few 'boarders may recall my previous posting on fuel economy. I've been tracking MPG by brand and octane since the car (2003 S60 manual transmission) was new. My latest sort of the data shows the following:

Synthetic Oil (Castrol Syntec 10w-30) 31.34 MPG over about 20,800 miles and 50 tankfulls of gas.
Conventional Oil (Castrol GTX 5W-30) 31.06 MPB over about 17,000 miles and 41 tankfulls of gas.

Conclusion: The occasional claims you hear of better fuel econonmy from synthetic oil are not supported in any significant way by these results. Kind of a no-brainer, but again facts are better than opinions. There may be benefits to using synthetic oil over the conventional stuff, but MPG is not one of them. The less than 1-percent difference, if it's not just a fluke of the data, in MPG would save you about $14 over 20,000 miles. Chicken feed.

Nearly all of my driving is on interstate highways in New England, including some extreme weather. I use Castrol products because they are specifically recommended in the Volvo manual. I do my own oil changes at least as often as the manual recommends.

Preliminary results on Sunoco: Sunoco falls into the "better brands" category so far. My previous posting showed a fairly significant difference between the "name brands" (except for Mobil) and the discount or "no names" with the higher price of name brand gasoline more than justified by better fuel economy. In short and somewhat counterintuitively, paying the higher price is actually cheaper. Also, I found no significant difference in MPG for 87, 89, 91, or 93 octane -- all about the same.