Chevrolet Express Owner's Manual page 268

Too much or too little fluid can
damage the 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. Too
little fluid could cause the
transmission to overheat. Be sure
to get an accurate reading if
checking the transmission fluid.
Wait at least 30 minutes with the
engine off, before checking the
transmission fluid level if the vehicle
has been driven:
In hot weather, when outside
temperatures are above 32°C
(90°F).
The vehicle is heavily loaded.
At high speed for quite a while in
hot weather.
In heavy traffic and hot weather.
While pulling a trailer.
After driving under these conditions,
a hot check can be performed. The
fluid should be hot, which is 71°C to
93°C (160°F to 200°F).
A cold fluid level check can be
performed after the vehicle has
been sitting for eight hours or more
with the engine off, but this is used
only as a reference. Let the engine
run at idle for five minutes if the
outside temperature is between
15°C to 32°C (60°F to 90°F). Should
the fluid level be low during this cold
check, the fluid must be checked
warm or hot before adding fluid.
If the outside temperature is colder
than 15°C (60°F) or hotter than
32°C (90°F), a cold check cannot be
performed.
A warm fluid level check can be
performed by driving the vehicle
under lightly loaded conditions and
outside temperatures between 10°C
to 27°C (50°F to 80°F). The vehicle
should be driven for at least 24 km
(15 mi) before performing a warm
check. Checking the fluid warm or
hot will give a more accurate
reading of the fluid level than a cold
check.
Because the vehicle is equipped
with a high-efficiency air-to-oil
cooler, the transmission fluid
temperature may not reach the
required hot fluid level checking
temperature under normal lightly
loaded driving vehicle conditions.
Prepare the vehicle:
1. Park the vehicle on a level
place. Keep the engine running.
2. With the parking brake applied,
place the shift lever in P (Park).
3. With a foot on the brake pedal,
move the shift lever through
each gear range, pausing for
about three seconds in each
range. When M is reached,
move the selector from M1
through M3. Then, position the
shift lever in P (Park).