It depends on your climate but, in most cases a 5W-30 is used.
5w30
Under the front, behind the bumper.
It is probably the oil cooler lines.
5 quarts 5W30
The crankshaft sensor on a 1999 Chevy Blazer is located on the bottom of the engine, near the oil filter. It connects to the vehicle's computer and constantly updates it on the crankshaft's position.
5w30
Passenger side firewall near oil fill for the motor
It will hold almost 3 pints.
4 and 1/2 qts with filter change.
I have always used 10w30. never had any problems.
The oil pressure switch on a 1998 Chevy Blazer is on the side of the engine block. It is located above the oil filter.
The oil filter is located below the front bumper, accessible through a door in the front skid plate.