GMC TopKick 2007 Owner's Manual page 281

How to Check and What to Use
Check the fluid level only when your engine is off,
the vehicle is parked on a level place, and the
transmission is cool enough for you to rest your
fingers on the transmission case.
To check the transmission fluid level, do the
following:
1. Remove the filler plug.
2. Check to be sure that the lubricant level is up
to the bottom of the fill opening. On heavy
duty transmissions, an inch of oil level equals
about one gallon of fluid.
3. If the fluid level is good, reinstall the plug and
be sure it is fully seated. If the fluid level is
low, add more fluid as described in the
next steps. Refer to the Maintenance
Schedule to determine what kind of fluid to
use. See Part C: Recommended Fluids
and Lubricants on page 397.
How to Add Fluid
To add transmission fluid, do the following:
1. Remove the filler plug.
2. Add fluid at the filler plug hole. Add only
enough fluid to bring the fluid level up to
the bottom of the fill opening. Refer to the
Maintenance Schedule to determine the proper
fluid to use. See Part C: Recommended Fluids
and Lubricants on page 397.
3. Reinstall the filler plug. Be sure the plug is
fully seated.
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