The oil can most likely go twice your old change interval, but the oil filter at oil needs changed at the manufacturer recommendations, 3000miles for example.
I recommend a high-mileage synthetic. 10w-30
100K
i had mine changed in Jan 2011 mileage 48000
Use the exact weight that is listed in your owners manual and nothing else. It can be synthetic, conventional, or high mileage.
120000km/75000m
The Chevrolet Silverado's gas mileage varies from 14 (in cities) to 20 (on the highway) EPA MPG. It is of course possible improve your mileage by, for example, changing oil. Most synthetic oils should help you improve you mileage.
80,ooo miles
90k miles.
no belt chain
yes your transmission fluid should be changed and should be changed every 80,000kms
No. If your using regular oil it should be changed along with the filter every 3000 miles. If you are using synthetic oil it should be changed with the filter every 5000 miles.
Should be changed every 60,000.