it should say in your owners manual.... but i believe it is rated to pull 17-18k lbs.... but i have seen guys pulling +25k with them.... either way, its your choice on how much you wanna pull... the more that you pull, the more chance you can damage the engine/transmission/differential.
Check your owners manual. The GVWR and the GCWR are the actual figures you should look at to calculate the weight of the trailer you want to tow. You can town more or less depending on how it hooks to your truck. As I calculate it when I put my 14,500 lb 5th wheel on my truck, which weighs 7500 lbs (single rear wheels) I am maxed out according to the GCWR. If you go over your load limits you can damage the vehicle and trailer, and it is unsafe.
18,000 lbs the only loads i can think of that u would need that much grunt to tow with would be comercial or farm hauling.
I towed 24k from out west to the east coast and went well. 7500km, no problem at all.
Looking for a Used F-350 Super Duty in your area?
CarGurus has 25,704 nationwide F-350 Super Duty listings starting at $1,995.
Search Ford F-350 Super Duty Questions
Ford F-350 Super Duty Experts