GMC 1998 Savana Van Owner's Manual page 258

Uses such as found in taxi, police or delivery service.
If your vehicle's GVWR is not over 8,600 and you
do not use your vehicle under any of these conditions,
change the fluid and filter every 100,000 miles
(166 000 km).
See "Scheduled Maintenance Services" in the Index.
How to Check
Because this operation can be a little difficult, you
may choose to have this done at your GM dealership
Service Department.
If you do it yourself, be sure to follow all the
instructions here, or you could get a false reading
on the dipstick.
NOTICE:
Too much or too little fluid can damage your
transmission. Too much can mean that some of
the fluid could come out and fall on hot engine
parts or exhaust system parts, starting a fire. Be
sure to get an accurate reading if you check your
transmission fluid.
Wait at least 30 minutes before checking the
transmission fluid level if
you
have been driving:
0
When outside temperatures are above 90°F (32°C).
0
At high speed for quite a while.
In heavy traffic -- especially in hot weather.
While pulling a trailer.
To get the right reading, the fluid should be at normal
operating temperature, which is 180°F to 200°F
(82°C to 93
O
C). See "Checking Transmission Fluid
Hot" in the Index.
Checking Transmission Fluid Hot
Get the vehicle warmed up by driving about 15 miles
(24 km) when outside temperatures are above 50°F
(10°C). If it's colder than 50°F (IOOC), drive the
vehicle in in DRIVE (D) until the engine temperature
gage moves and then remains steady for 10 minutes.
Then follow the hot check procedures.
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