Shape meat into six patties; place in a shallow dish. In a small bowl, whisk together the oil, soy sauce, ketchup, vinegar, garlic and pepper. Set aside 1/4 cup for basting; cover and refrigerate. Pour remaining marinade over the patties. Cover and refrigerate for at least 3 hours.
Grill burgers, uncovered, over medium heat for 5-6 minutes on each side until a thermometer reads 160° and juices run clear, basting occasionally with reserved marinade.
Serve on hamburger buns with lettuce leaves and tomato slices if desired.