I want to cook eggs and I want to know the amount of oil that I have to add for 4 eggs. The eggs' weight is about 75-80gr.
Assuming you are talking about fried eggs, use a non-stick pan, and use just enough fat to coat the pan. If you're using an oil, don't measure, just use a few drops, and make sure to spread it thinly on the pan. You could also use cooking spray. The key is having a really, really slick non-stick pan. No pan with any kind of texture will do.
For scrambled, the same is true, but I'd suggest using butter, and just a little more than you need. For 4 eggs, I'd use about a teaspoon of butter. Of course, I also add whipping cream to my scrambled, do it over a double-boiler, and often finish with a small amount of melted butter on top as it is. Here, you want to really keep the eggs moving to keep from sticking.
If you're talking about another cooking method, please be a little more specific.
if you are using olive oil, use little. Perhaps a half spoon, at any rate enough to grease all the bottom of the pan but not enough to make a puddle. If you use too much it will make bubbles at the edges of your eggs and spatter everywhere, but mostly on your hands and shirt.
Where frying eggs is concerned, an important element, almost as essential as how much oil you use, is that the pan is heated and the oil is at the right temperature. Be sure to heat the pan and oil to a point where (1) the oil becomes very fluid and there is just a negligible amount on the bottom of the pan to swirl around freely and (2) the oil should spit if you flick some water in it.
I have a 12" anodyzed pan and it only needs about two teaspoons vegetable oil. If it's the kind of thing that concerns you, I find you can get away with less when you use peanut or sesame oil.