GMC SIERRA CK2 1999 Owner's Manual page 380

Short Trip/City Scheduled Maintenance -- Gasoline Engines
50,000 Miles (83 000 km)
j
Change automatic transmission fluid and filter if the vehicle's GVWR is over
8600 lbs. or if the vehicle is mainly driven under one or more of these
conditions:
– In heavy city traffic where the outside temperature regularly reaches 90 _ F
(32 _ C) or higher.
– In hilly or mountainous terrain.
– 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 at 100,000 miles (166 000 km).
Manual transmission fluid doesn't require change.
51,000 Miles (85 000 km)
j Change engine oil and filter (or every 3 months, whichever occurs first).
An Emission Control Service.
j Lubricate chassis components (or every 3 months, whichever occurs first).
(See footnote #.)
j Check rear/front axle fluid level and add fluid as needed. Check constant
velocity joints and axle seals for leaking. (See footnote **.)
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