I'll second what everyone else says. The reason your dealer recommends normal change intervals even when using syn oil is probably due to perceived warrentee issues (though you are out of warrentee), and he likely assumes that you will use an off-the-shelf synthetic like Mobil 1, etc. that have standard additive packages. When you use Amsoil, Neo, or Redline, you can indeed extend the interval safely. I go about 23K miles on Amsoil 10W30 in my Jeep, and about 10K miles on my S70 T5, and one year on my Volvo-Penta boat engine, with no problems what so ever. I always do an analysis on the Jeep and Volvo, and they always turns out fine.

Premium syn. has superior and expensive additive packages that give them long term durability. They generally have a very high Total Base Number, indicative of the oil's ability to neutralize corrosive combustion byproducts; lots of dispersants, to keep pollutants in the oil and not on your oil pan; lots of anti-wear additives, etc.

A major benefit of using synthetics in your Turbo is that true synthetics are very heat stable, and will not coke in the turbo. For the longevity of your turbo, this is very important. In my previous 740 T, even after 190K miles, the turbo operated like new.

I highly recommend you switch to a good syn. like Amsoil, which, based on my researh is the best. Mobil 1 is also very good, but will require standard change intervals. You can order Amsoil straight from their web site, or better yet, order it from one of our forum dealers.
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1999 S70 T5