Volvo allowed the change in '04 as a result of the outcry from the field over the outrageous cost of the OEM synthetic fluid. There is a very specific bulletin on it from Volvo.
AW55 transmissions were/are used in many maker's lines. But just because AW says something generic doesn't make it applicable to the individual manufacturers. Volvo mated these to engines more powerful (including turbos) than some others did.
Transmission families get refined and less trouble prone across their production lives, so I would think that for the later production the fluid spec becomes less necessary to cover other design issues. The classic instance of this was the early Taurus four speeds, which routinely self-destruct from overheating. Now, the right way to keep those running is with bigger aftermarket coolers and synthetic ATF, even though FoMoCo always recommended conventional ATF in them.
For the earlier AW55s, I would try to stick with the synthetic OEM fluid, especially in a turbo or if he doesn't plan very regular changes. The Volvo OEM bottled fluid is synthetic, and admittedly costs a lot. It's still a lot cheaper than a rebuilt AW55.
And just because the dealer pumps it in doesn't make it the best choice. They also pump bulk conventional into Rs and other engines for 7500 mile runs (at Volvo's recommendation), and now sludging reports are increasing. I can understand the widespread use of the Mobil 3309. It's a lot cheaper. But frankly, there's a lot of dealers out there that don't give a rat's backside if the engine or transmission blows outside the warranty. That's how they herd them over to the sales floor to buy a new one.
This is how it has been with Volvos. We all went through the same thing with the 850s. The early ones had their issues, and you need to be more diligent in maintaining those. By the end of the model run, the transmissions (and most of the rest of the car) were pretty much bulletproof.
This guy has an early S60. That was the specific reason for the caution on the trans fluid, despite what the dealers might be pumping. Of course, he can run whatever he wants.
