what fuel type for silverado 1500

I suggest following your owners manual info. I don't know, who are " they"? I did look at your owners manual, I don't know if your 1500 or 2500, The lead in for your post shows both? I looked at 1500 model. If not flex fuel, don't use fuel with 15% or greater ethanol, so says that owners manual. Different parts of the country may have different blends? Talk to the service department of your local dealership, ask them about it?

2018 should be flex fuel. the 5.3 ltr V8 runs well on regular 87 octane gasoline as does the 4.3 ltr V6. Older engines don't work well with the ethanol (E85) because ethanol can eat seals in the engine causing it to burn oil. If you have the 2500 with a 6.6 ltr diesel, well, use diesel. You will be able to tell. If you can't fit anything under the hood and you have two batteries, its a diesel.