Brush both sides of each slice of bread with the truffle oil or butter. Place a slice of cheese on top of each 4 slices of bread, then top with the remaining slices of bread. Grill the sandwiches on a hot griddle or in a nonstick skillet over medium heat until golden and the cheese has melted, 2 to 3 minutes on each side.
Cut each sandwich into 5 finger-size slices to serve.
Yield: about 10 servings