GMC 1999 Envoy Owner's Manual page 341

Long Trip/Highway Scheduled Maintenance
– When doing frequent trailer towing.
– Uses such as found in taxi, police or delivery service.
If you do not use your vehicle under any of these conditions, change the fluid
and filter every 50,000 miles (83 000 km).
j
Rotate tires. See "Tire Inspection and Rotation" in the Index for proper
rotation pattern and additional information. (See footnote +.)
22,500 Miles (37 500 km)
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Change engine oil and filter (or every 12 months, whichever occurs first).
An Emission Control Service.
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Lubricate chassis components (or every 12 months, whichever occurs first).
(See footnote #.)
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Check rear/front axle fluid level and add fluid as needed. Check constant
velocity joints and axle seals for leaking. (See footnote **.)
j
Rotate tires. See "Tire Inspection and Rotation" in the Index for proper
rotation pattern and additional information. (See footnote +.)
30,000 Miles (50 000 km)
j
Change engine oil and filter (or every 12 months, whichever occurs first).
An Emission Control Service.
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DATE
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