Heat a charcoal- or wood-fired grill to medium (350°; you can hold your hand 5 inches above cooking grate only 5 to 7 seconds).
Meanwhile, cut four sheets of foil, each about 1 by 1 1/2 feet. Set a salmon fillet on center of each. Drizzle each salmon fillet with about 2 teaspoons oil, sprinkle with salt and pepper, and turn to coat. Then drizzle each with 2 tablespoons of the Homemade Ranch Dressing and turn to coat, ending with skin or skinned side down. Seal foil lengthwise, folding edges together several times. Roll up ends of packets close to fish.
Set packets on cooking grate folded side down and cook until you hear sizzling, about 3 minutes. Using grilling tongs, turn packets over. Carefully slit foil lengthwise with a small knife. Steady packets with knife and open them up with tongs, without spilling sauce. Cook until fish is just cooked through, 1 to 7 minutes more.
Tear dill sprigs into pieces and scatter over fish. Serve with more dressing on the side.