Set oven to 325 degrees F, convection oven heat set to 320 degrees F.
Set oven rack to lowest position.
Remove the giblets and neck from the cavity.
Rinse the turkey well with cold water inside and out then pat dry with a paper towel.
Place the turkey on a rack on a roasting pan.
In a small bowl mix together the melted butter with oil (I also like to add in 1 teaspoon garlic powder with the oil/butter mixture if you are a garlic-lover then use it).
Fill the inside cavity loosley with prepared stuffing (if you are not using stuffing then tie the legs together with cotton butchers twine).
Using clean hands rub the outside of the turkey with the oil/butter/garlic mixture.
Season the turkey all over with salt and lots fresh ground black pepper.
If you are NOT stuffing the bird place breast-side down on the rack, if stuffing leave breast-side up.
Place/tent a large piece of heavy foil over the turkey.
Place the pan in the oven, then pour about 4-5 cups chicken broth in the pan.
Keep basting the turkey about every 30-40 minutes with the pan juices on the bottom of the pan (removing the foil and re-tenting every time you baste).
If you see the juices starting to evaporate then add in more broth to the pan about 2 cups at a time.
After about 2-1/2 hours of cooking time remove the foil.
Keep roasting (325 degrees F) until the internal thermometer reads 180 degrees F (80 degrees C).
The total cooking time for this 18-pound stuffed bird should be about 4 hours.
Transfer the cooked turkey to a large serving platter, cover loosley with foil and let rest 30 minutes before carving (DO NOT slice the turkey before 30 minutes or all juices will flow out of the bird!).
**NOTE** cooking times for whole turkeys cooked at 325 degrees are as follows.
For an 8-12 pound turkey, roast for 2-3/4 to 3 hours.
For a 12-14 pound turkey, roast for 3 to 3-3/4 hours.
For a 14-18 pound turkey, roast for 3-3/4 to 4-1/4 hours.