GMC 1999 Sierra 1500 Pickup Owner's Manual page 306

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Service & Appearance Care
Just fill it enough to put the level somewhere in the proper operating range.
Push the dipstick all the way back in when you're through.
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What Kind of Oil to Use:
Look for these two things:
CElSG
K2396
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For SAE 15W-40 or SAE 1OW-30, CE must be on the oil container. For
SAE 30, CD must be there. These may be listed alone or with letters,
such as CEISG, CE/SG/SF, SGICD, etc. These letters show the
American Petroleum Institute (API) levels of qualijr.
NOTICE
If you use oils that don't have these designations, you can cause engine
damage which is not covered by your warranty.
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