What is the oil used in a 1993 Chevy silverado 3500 6.5 turbo diesel?
The recommended oil for a 1993 Chevy Silverado 3500 with the 6.5L Turbo Diesel engine is 15W-40 diesel engine oil meeting the CF-2 specification (or better). GM specifically recommended this viscosity and specification at the time.
However, you should always check your owner's manual for the most definitive answer and any specific brand recommendations. The manual will also specify the correct oil capacity. Using the incorrect oil can severely damage the engine.