USE EITHER NATURAL 10W30 MOTOR OIL OR SAAB'S SYNTHETIC, PN 87 48 733. DO NOT MIX THEM.
UPDATE: I HAVE AN EXCEPTION! Tom Townsend recommends Amsoil 20/50W Series 2000 synthetic in both 900 and 9000 5-speed transmissions. He has been using this stuff for some time now, and he absolutely swears by it and has many glowing success stories. Since he was so high on it, I put it in my transmission after having it rebuilt. All I can say is that after about a year I've had no problems with it.
5-speeds made before 1994 did not have drain plugs. Level is checked by a dip stick on the front of the transmission. Oil can be sucked out of the dipstick hole using a hand-held siphon pump, but only about 60% of the oil can be removed this way.
From 94 on, level is checked by removing a plug, and a drain plug is also provided. The level and drain plugs are on the left end of the transmission, visible by removing the left front tire and the front of the inner fender liner.
Saab says it is not necessary to change the manual transmission oil, ever. Saab originally specified only 10W30 natural engine oil for 5-speeds, but they subsequently developed a synthetic (PN 87 48 733) which, presumably, they now recommend.
Some people say that Redline MTL works fine, but there have been entirely too many reports of transmission failure shortly after installing Redline MTL. In my opinion, you should not run MTL in the Saab 9000 5-speed. And you absolutely should not run hypoid gear lube in it either. As stated above, use either natural 10W30 motor oil or Saab's synthetic, PN 887 48 733. Do not mix the two.
Can I bore you with my "better ain't always better" lecture?
There are pros and cons about switching to the synthetic and, in any case, there's a Saab procedure for making the switch; details.