Drizzle swordfish steak with lemon juice and let stand for 1 minute. Rub both sides of each steak with olive oil, salt and pepper and set aside. Place 4 tbsp unsalted butter, softened, 1 tsp freshly squeezed lemon juice, lemon zest from 1 lemon, 1/2 tsp garlic, very finely chopped, 1/4 tsp salt and 2 tbsp minced fresh basil in a small bowl and combine well. Set aside until ready to serve. Oil the grill grates and add the fish. Grill the swordfish steaks until opaque throughout, 3-1/2 to 4-1/2 minutes per side. Top each piece of fish with about 1 tbsp of the lemon-basil butter just as it comes off the grill. Allow to rest for 2 to 3 minutes before serving.