I'm going to deal with your questions separately, since they really aren't related to one another.

Today he hasn’t eaten or drank water, all he does is sleep. We had a houseful of kids here yesterday, could that be a factor in his behavior today?

Going without food for a day really isn't something I'd worry about. Just like people, dogs’ moods and appetites fluctuate. As long as he's not losing weight, I wouldn't worry too much about him. He'll bounce back to normal in no time. That said... Water is an entirely different matter. His liquid intake can drop into the danger zone within one day. Watch out for signs of dehydration. Gently pull a bit of skin away from his body, does it fall right back into place or stay roughly where you moved it to? If he's got enough fluids it should fall back into place as soon as you drop it. If it doesn't, I'd call your vet for advice. It could be nothing, but he may need intravenous fluids.

The vet told me to give him Beneful dog food

Wow! I have to admit, that's a real shocker to me. Amongst dog fanciers Beneful has a nickname - "Benecrap". There really are MUCH better food choices out there. I'm extremely surprised to hear that any vet would recommend it. Take a read through my article on deciphering dog food labels.  It should help you make the right choice for your boy.

Will oil hurt everyday?

Definitely not! However, by switching him over to a better food he shouldn't need the oil any longer. I'd be interested in hearing why your vet recommended Beneful with oil. Generally oil is added to help with the coat, or for problems with dry skin, but (as you've noticed) it can also help stimulate appetite... so either stick to your guns, if he doesn't eat that day, he doesn't eat... or start buying stock from your dogs favorite oil company.