Spark plug gap for 2002 Chevy Avalanche z66

Hi so I went to the store today to buy spark plugs and the guy who helped me seemed to know about car stuff pretty good. I bought ac delco , I don't have the make name but they were 5 or so each. The guy said they were pre gapped to .45 and I checked the manual it said .60 , so I went back in and he was nice about everything he didn't force me to buy anything so when I said that he pulled up in the screen that the "original " we're set to .40. When I said .60 he seemed surprised like it was a lot. So there in the truck right not at .45 is that bad ? Should I leave them or do them again ? And can I put dielectric grease on the other side of the boot  ? Or none is needed there

Always follow the manual.

Never trust a set of plugs being pre gap, they could had been dropped during shipping.

Dielectric grease is good just don't get crazy with it, a very light coat on the inside of the boot

I figure out the name. They are the ac delco rapid fire (5531160). So I should take them out and gap them  to .60 ? When do you think I should change them , I think I heard 60,000 ? But not sure.  Thank you

I would not bother with that, leave them in.

It will run fine with the plugs gapped at .045, but with a gap at .060 will ignite the fuel better and get a more complete burn. It's what the factory calls for so that's what they should be gapped at.

As far as when to change or service interval. GM says they can go 100K, but you will see and feel an improvement if you change them sooner.. I change mine every 50K..

So does that mean I will actually be losing mpg ? Since it won't burn as good ? Well if you look in the manual. It's confusing. It's like another language hah. I just see .60

Probably would be hard to measure unless you ran half a dozen tanks with it at .045 and the re-gapped at .060 and ran a half dozen more, but over the long run, yes. Since there already at .045, why don't you do that and keep detailed record of your mileage and let us know if there's a difference.

Hey now I'm not gonna be the  Guinea pig lol. Plus to late I already re gapped them. Haven't driven it yet. Will let yall know if I feel a difference at least hah