What transmission oil do I use for my 1997 saab 900 se turbo manual transmission?

Saab, engineered and built your car. If it says 10w30 for the transmission in the owner's manual why would you come here and ask what should be put in the transmission? Put in what the manufacturer recommends.

I didn't finish my question, when looking online before I found the manual it says they now recommend a synthetic, but not to mix them. I just bought the car and don't know what the previous owner put in. Can I drain the fluid that is already in just to be safe?

You could probably drain it and let it drain really well then refill with the 10w30 or use the later recommended synthetic. It would be difficult to flush all of the lubricant out of the transmission. Draining and leaving the drain plug out for several minutes or longer will get most of the old oil out.

cant find gearbox plug to change oil