Best Octane Gas

Mechanics and Owners manual say the Regular Gas. Which is best to use without harm: Regular, Mid-Grade, or Supreme?

I run a 91 Octane which is "Supreme" in Canada. I will try a lower grade and get back to you.

I have an '01 with the 5.3 and have been running regular with no problems since it was new.

Can´t say, I run propane, I only fill up four times before I have to change the oil, it´s a beautiful thing.

Your 99 is designed to run on Regular, do not be fooled by ads. If you had a high compression engine or fast-burn heads, you would need high octane. You just need to get your gas from a reputable company from a gas station that sells a lot and keeps its equipment maintained. Your worst enemies are sediment, rust, and stale fuel. Unless your manual says to run high octane, don't. Your gas mileage may even go down if you use Ultimate. I run Regular in my 96' and ocassionally use SeaFoam to clean my fuel system. I have had no undesirable running. If you plan on taking your truck to the track and you have a turbo or supercharger or nitrous, then I would highly suggest that you use high octane. Other than that, don't waste your money. -Chris

Or if you get a tuner that bumps the timing up, switch to 93 octane, or the valves will rattle.

The best gas to use in your truck is the lowest octane level that doesn't cause frequent knocking (think rattling marbles) or pre-ignition (engine still runs after you turn off the ignition). I have a '99 'Rado 5.3 and have never had problems using 87 octane, even on steep hills. What's more important is that your fuel filter is changed periodically as well as your injectors cleaned. I buy my gas from RaceTrac, Costco, Wal Mart, etc as opposed to the $.15 more per gallon Shell, BP, etc, using the rational that you can get bad gas no matter where you buy it. Just make sure your fuel line components are changed/cleaned periodically. That, and changing your oil & filter every 3-5K miles, and you'll take care of most of your potential major problems. I speak as an owner of 15 new (all but 2) GM cars that I've kept from 8 to 10 years each.