The type of engine oil a 1999 Chevy Silverado takes depends on the engine it has. Silverados of that year came with several different engine options. To know for sure, you need to
check your owner's manual. It will specify the recommended oil type (e.g., 5W-30, 10W-30) and the API certification (e.g., SM, SL).
However, common engine oils used in 1999 Silverado's were typically 5W-30 or 10W-30. But again, consult your owner's manual for the exact specifications. Using the wrong oil can damage your engine.