As far as a turbo goes, just use a good quality synthetic. The only special considerations a turbo adds to the equation are more heat (typically higher exhaust gas temps) and the oil-lubricated floating bearing in the turbo itself (I'm pretty sure the S40 doesn't use a ball bearing turbo). Anyway, turbos usually fail when that center bearing's seals deteriorate due to sludge buildup (coking) or what have you narrowing the oil passage and increasing pressure pushing oil thru the seals and into the intake track. With a modern water cooled turbo, this is much less of a problem.
I'm not sure about the GC, but just use a quality synthetic and stick to a traditionally short change interval. SAAB is having problems right now with late 90's thru 2003 models sludging and killing turbos due to pcv problems exacerbated by the factory-recommended 10k mile change interval. I did have a 97 900 and did short intervals w/ M1 and never had a leak or a bit of sludge. However, I just got a used '01 9-5 which appears to have been victim to the 10k changes and it is lightly sludged at roughly 1/4 the mileage of my clean 900. Turbo'd cars make more heat and need a bit more consciencious care.
As Saab proved, the manufacturer doesn't always know best. Now they're having to provide 8 year extended warranties on all these 9-5 2.3l engines.