Sorry here is the body of the comments
I say use synthetix whenever you feel like it.
But be prepared to lose some compression as the synthetic oil cleans your engine. Using petroleum oil creates sludge build-up that actually helps retain cylinder compression. Switching to synthetix has a neat way of clearing out that sludge and the compression value created by sludge.
It's better to switch as soon as possible rather than allow the engine to become dependant on sludge simply to have compression.
I use Amsoil Euro 5w30 and Tufoil engine additive. I switched my XC70 at 52k about three months ago when I purchased the car.
It runs like a dream and MPG increased from 18.3 to 26.
Not all synthetix are better than petrol oil. Avoid Pennzoil like the plague.
Castrol Syntec is very good, but burns off easy. It cleans extremely well.
I always recommend Amsoil and Tufoil. Amsoil was the first to sell automotive synthetix back in 1972. They know it better than anyone else.
Tufoil holds the Guiness World record for being the slipperiest (sp?) substance known to man. In a test against 60 other oil additives in the four-ball wear test, all other products burned up into smoke under 6 minutes. Tufoil lasted 16 days.
With synthetix, your main concern will shift to how good your filtration is, more so than oil longevity.
Good luck.
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A photo of my '02 XC:
http://i72.photobucket.com/albums/i1...o/DSC00204.jpg