1994 GMC Suburban Owner's Manual - Page 271

DIESEL ENGINES SELECT THE SAE GRADE OIL BASED O N THE EXPECTED TEMPERATURE RANGE BEFORE NEXT OIL CHANGE HOT WEATHER :F t 100 ;C - +38 +60 - - + 1 6 +32 -- 0 I I SAE 30 0 - - -18 I -20 - - -29 SAE 1OW-30 SAE 15W-40 PREFERRED above 0°F (- 18°C) COLD WEATHER DO NOT USE SAE 1OW-40, SAE 2OW-50 OR ANY OTHER GRADE OIL NOT RECOMMENDED SAE 15W-40 As shown in the viscosity chart, SAE 15W-40 is best for your vehicle. However, you can use SAE 1OW-30 if it's going to be colder than 32°F (OOC) before your next oil change. When it's very cold, below 0°F (-18"C), you should use SAE I OW-30 to improve cold starting. Also, SAE30 may be used at temperatures above freezing, 32°F (0°C). These numbers on the oil container show its viscosity, or thickness.Do not use other viscosity oils such as SAE 10W-40 orSAE 20W-50. 6-25