I purchased new a 2000 GMC Sierra with Vortec 5.3 V8. Here is my oil consumption, converted to U.S. quarts and miles:
Mobil 1 0W-30 1 qt every 4,200 miles
Mobil 1 5W-30 1 qt every 4,600 miles
Mobil 1 10W-30 1 qt every 6,800 miles
Mobil 1 15W-50 0 after +7,000 miles
So the basestocks and additive packages do make a difference. I tried the 15W-50 in summer of 2001 doing heavy towing in hot temps. The motor was QUIET. Also, when empty there was NO difference in fuel economy compared to the 0W-30 or 5W-30.
Running either 0W-30 or M1 5W-30, I get a lot of tick-tick-tick valvetrain noises at operating temps. Most of these noises go away with M1 10W-30 and are completely gone with M1 15W-50.
My winter temps can dip to -42 so I absolutely recommend the M1 0W-30 at those temps. It will save the motor. However, in summer with temps +85 F and heavy towing, the motor is VERY noisy on 0W-30.
When I change my oil the end of this April, I'm going back to 15W-50.
Jerry