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My manual does not tell where to buy the fluid except overpriced at the Chevy dealer.
To get the best price you want to order it with the AC Delco PN not the Chevy part number. Both PNs are on the bottle and Summit Racing charges $20 more per bottle if you order using the Chevy PN rather than using the ACDelco PN.
My local dealer charged me $18 for 1 quart of Mobil 1 Supercar 5W-50 oil last week (not the OPs question, thought).