According to Amsoil.com, 1999 OLDSMOBILE ALERO 3.4L 6-cyl Engine Code [E] preferred oil type is 5W-30 (All temps); for above 0°F preferred motor oil is 10W-30.
As per Carcarekiosk.com, most Oldsmobiles have the oil type printed on the oil cap - it will likely be 5W-20, 5W-30, 5W-40, 10W-30 or 10W-40. If it isn't printed on the oil cap, check your owner’s manual for the exact type before adding new oil.
Make sure to wear safety glasses whenever you are working under your Alero. As you'll never know what could be dripping down from the engine, battery acid, engine coolant, brake fluid, etc. All of these fluids are extremely harmful to your eyes and skin so it is important to protect yourself.
If you want to own and properly maintain a vehicle, at some point you'll have to figure out what kind of oil your car takes. Whether you're at a service center being asked what kind of oil you want or at a parts store shopping for oil to top off the engine, you'll want to know what's recommended.
Check out some tips on how to choose the right engine oil at Cars.com.