You can adjust the timing. You're not supposed to but you can. Just make sure you understand what you are doing and you can cause detonation among other things if you go too far for your engine.
You have to understand why you change the timing. You probably understand that an air/fuel mixture goes into the cylinder and is ignited and that pushes the piston down. The problem is that it takes a little time from when you ignite the mixture till it develops full power. You want the piston to be pushed down immediately after top dead center but if that's when you ignite the mixture then the piston will already be going down before you reach full power. So you have to fire the plug before TDC. There are lots of variables that go into exactly when like fuel type, octane, weather, altitude, normal variations in mass production tolerances of the engine, etc. So Ford picked 10* as a safe number that can be used in all trucks with the minimum available grade fuel, etc. 10 is very close to ideal but for those of us that can't leave well enough alone you can advance the timing farther to see if you pick up more performance. But if you go too far you'll be hitting the piston before it gets to TDC and you'll be fighting the engine. You can also damage the engine this way. So you have to be careful and you have to understand that you can't just set your timing at 13* because someone else did. You have to listen to your truck and make sure it's not showing any signs of detonation - that rocks in a coffee can sound. And it's not going to be full on rattle, rattle, rattle, it's going to be a ping here and a ping there. You have to listen close and as soon as you hear just one ping you need to back off the timing.
So 13 is a guideline, not an absolute. I like to sneak up on it and see how it runs. I like to advance 2* at a time and run the truck for a little making sure to cruise, and also romp hard on the truck. I keep going 2* at a time till I hear it ping then I back off 1*. If you're not sure you're catching it right on that edge you can back off 2* to be safe. Remember, overly advancing your ignition timing can damage your engine.
With that all said the last time I had a timing light on my 89 5.0L Bronco it was at 13*...
