Make adobo: Slit chiles lengthwise, then stem and seed. Heat a large heavy skillet (not nonstick) over medium heat until hot, then toast chiles in batches, opening them flat, turning and pressing with tongs, until more pliable and slightly changed in color, about 30 seconds per batch.
Transfer to a bowl and cover chiles with boiling-hot water, then soak until softened, about 15 minutes.
Meanwhile, toast spices in a small heavy skillet over medium-low heat, stirring frequently, until fragrant, about 3 minutes.
Drain chiles, discarding liquid, and purée in a blender with spices, garlic, herbs, vinegar, water, oil, and 2 teaspoon salt until very smooth, about 1 minute. Set aside 1/2 cup adobo for gravy.
Marinate turkey: Rinse turkey inside and out and pat dry. Sprinkle 2 teaspoons salt evenly in turkey cavities and all over skin, then rub remaining adobo (a scant 3/4 cup) all over turkey, including cavities. Fold neck skin under body, then tuck wing tips under breast and tie drumsticks together with string. Transfer to rack in roasting pan and marinate, covered with plastic wrap and chilled, at least 8 hours and up to 24.
Roast turkey: Let turkey stand, covered, at room temperature 1 hour.
Preheat oven to 350°F with rack in lower third.
Add 1 cup water to pan and roast turkey 1 hour.
Brush turkey with oil and add remaining cup water, then tent loosely with foil and rotate pan. Roast (if bottom of pan becomes dry, add 1/2 cup more water) until an instant-read thermometer inserted into fleshy part of each thigh (test both; close to but not touching bone) registers 170°F, 1 3/4 to 2 3/4 hours more (total roasting time: 2 3/4 to 3 3/4 hours).
Carefully tilt turkey so juices from inside large cavity run into pan. Transfer turkey to a platter and let stand, uncovered, 30 minutes (temperature of thigh meat will rise to 175 to 180°F).
Make gravy while turkey stands: Straddle roasting pan across 2 burners, then add 1 cup turkey stock and boil over high heat, stirring and scraping up brown bits, 2 minutes. Strain pan juices through a fine-mesh sieve into 2-quart measure and skim off fat (or use a fat separator), reserving fat. Add enough turkey stock to liquid to bring total to 5 cups.
Whisk together flour, 6 tablespoon reserved fat (if there is less, add melted butter), and reserved 1/2 cup adobo in a heavy medium saucepan, then cook over medium heat, whisking constantly, 3 minutes (mixture will be thick). Add pan juices and stock in a fast stream, whisking constantly to prevent lumps. Bring to a boil, whisking, then simmer, whisking occasionally, until thickened, 10 to 15 minutes. Season with salt.