Croutons, like aubergines, will soak up as much oil as you give them. You have to impose a ration, insisting that they make do with it. One way is to toss the croutons in oil, lay them on a baking sheet, and put them in a hot (gas mark 6/200 C) oven for five to 10 minutes; but I also fry them. For two slices of cubed bread (crusts removed), I warm 2 tbsp olive oil in a saucepan on a medium flame, throw in the cubes, and toss them gently. They suck up the oil instantly: I stir them round the dry pan until they are crisp.
The croutons I made for lunch yesterday (for two) disintegrated a little, giving me a mix of cubes and crumbs. That did not matter. I washed and tore up some lettuce, and cubed about 150 g Gruyere; I tossed them in a vinaigrette made with 1 dstsp red wine vinegar and only 2 dstsp olive oil (because we were also getting the fatty contents of the cheese and of the croutons). Plus a little salt, and generous grindings of pepper. I scattered the croutons on top.