Buick RIVIERA 1993 Manual page 231

Automatic Transaxle Fluid
When to Check and Change:
A good time to check your automatic transaxle fluid
level is when the engine oil is changed. Refer to the
Maintenance Schedule to determine when to change
your fluid. 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 a Buick 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.