Semisyns include both synthetic oil and natural. Synthetic oils are better able to handle the stress of the turbocharger; while the natural components have a greater tendency to coke under high heat high stress (turbo) and the result could be lack of flow. Saab included Semi-syns originally thinking they were acceptable; but then they ran into trouble from it and the newer cars require only full synthetics.
On older T-5 engines there is some room for play as far as what to use; but you should stick with a full synthetic especially if you have any plans to tune it. Any good synthetic oil will do what you need; just get the right weight for the temps in your area. Any good Full syn will be fine for the T-5 engines; including Mobil 1; Shell Rotella T-Synth, Valvoline, Castrol etc.
T-7 engines are a different story though. The revised engine and increased sludging issues require more care for T-7s and a more cautious oil choice. Here is a list from that other
site that spells out some good quality oils for the T-7:
Here's a list of motor oils that are either ACEA A3 certified (Saab's requirement for 2000-2002 9-3's and 9-5's plus the Viggen) or are otherwise known to be stout enough to handle the harsh environs in the potentially sludge prone B205/B235 engines. This list is based on US oils. If you are outside the US and have a T7 car, look for the ACEA A3 spec on the back of the oil container.
Brian's Recommended Oils for T7 SAABs
AMSOIL Euro Formula 5W40
Shell Rotella T Synth 5W40
Valvoline Maxlife Synthetic 10W30
Valvoline Synpower 5W40
Valvoline Synpower 20W50
Castrol Syntec 0W30 (only available at Autozone)
Castrol Syntec 5W40
Castrol Syntec 10W40
Castrol Syntec 5W50
LubroMoly VollSynthese 5W40
Mobil1 15W50 EP
Mobil1 High Mileage 10W40
Mobil1 TurboDiesel 5W40
Mobil Delvac1 5W40 (mainly at truckstops)
Pennzoil Platinum Euro Formula 5W40 (hard to find)
Personally, I use a High quality 5w40 full synth for my 02; year round. I am installing oil pressure And oil temp gauges shortly so I can monitor; but the data so far from others shows this weight to be safe year-round unless you live where there are temp extremes. I'll do a write up and post pictures as soon as I get the gauges done
As far as the viscosity numbers; they are oil "weights" to let you know what the oil will be like. a 10w30 for example is giving you the weight and flow characteristics of the oil cold (10) and hot (30). Colder temperatures require a quicker flowing oil for cold starts; that is why the manual tells you to use a 5w instead for those places.
Take a look at Bob is the Oil Guy
for great info on oil; how it stacks up; what works and why; and pretty much anything you ever wanted to know about oil.