Chevrolet 1998 C/K Full-Size Pickup Owner's Manual page 379

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Short TripKity Maintenance Schedule
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Gasoline Engines
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Drive axle service (see "Recommended Fluids and
Lubricants" in the Index for proper lubricant to use):
0
Locking Differential
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Drain fluid and refill at first
engine oil change. At subsequent oil changes, check
fluid level and add fluid as needed. If driving in
dusty areas or towing a trailer, drain fluid and refill
every 15,000 miles (25 000 km).
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Standard Differential -- Check fluid level and add
fluid as needed at every oil change. If driving in
dusty areas or towing a trailer, drain fluid and refill
every 15,000 miles (25 000 km).
More frequent lubrication may be required for
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3500 HD Models with applications requiring
heavy-duty or off-road use.
extreme overload/trailer towing conditions and
high-speed (above 45 mph or 70 k d h ) conditions
for extended periods of time must have the drive axle
fluid changed every 30,000 miles (50 000 km).
See "Recommended Fluids and Lubricants" in
this section.
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