I just bought a 1989 900 Turbo. The owner didn't have the car's owner's manual, so I've been searching for what oil weight to use. I can't seem to find anything on it.

What oil weight should i use?

I'm pretty sure the owners handbook says 10 W 30 or thereabouts. 5 W 30 is fine too. I use 5 W 40 with no problems. Fully synthetic is better for turbos. Used to use Genuine Saab, then Mobil 1, now some Vauxhall branded stuff. Change every 5000 miles, less if you prefer.

Lots of stuff on engine oil here. (http://www.chris-longhurst.com/carbibles/engineoil_bible.html)