Hard to get in gear intermittently

[Chamberpains]

It's called the flywheel housing gasket (self explanatory that it houses the flywheel). The bell housing is on the transmission. On the ISX they use a string style gasket and they have a tendency to leak on high mileage engines and engines that use high torque a lot. If your truck has more than 600,000 miles on it then it's just good practice to go ahead and replace that gasket when you do the clutch. If you don't you stand a very high chance of that gasket starting to leak in the next few hundred thousand miles or sooner and then you'll just be pulling it all back apart to do it. It's better to have it done now.