Eventually - all of them. You'll see.
The most important are the fuel lines - the air sucking issues on the low pressure lift pump side are death for the 3126 and C7 HEUI pump. Also important for the 3116 to protect the fuel pump and injectors since they aren't easy to source anymore and are usually back-ordered from CAT. Air in the fuel leads to cavitation that damages the high pressure components at a microscopic level.
All the air brake, CTIS, and breather hoses for each wheel. Especially in the front where they like to abrade each other due to the tight bundling and moving against each other from being old and hard and the fabric picks up dirt and turns sun-rotten and filled with abrasives so they chew holes in each other.
The hydraulic lines often lose significant portions of their outer sheathing so end up with no protective layer and are easily damaged. If you don't want to get stuck without being able to lift the cab to fix something minor (will happen) or get a flat and find your spare crane doesn't work I would suggest replacing the lines, rebuilding both pumps, and rebuilding the cab latch are all very important.
ALL the rubber will fail. The CAT 3126 and C7 need every pressure transducer replaced - the rubber inside them that seals the strain sensor against hot fluid intrusion eventually fails and they stop reading. go open circuit, or go short - or all three depending on the time of day and the location of the moon and the stars and sometimes they just can't make up their mind how they feel and switch it up every few seconds. Fun.
Same goes for all the weather stripping, tires, and anything else that's rubber. Fortunately we don't have much plastic although the fenders get pretty gnarly when they are exposed to hot beating sun for a decade or so. Had to replace both of mine due to pretty severe bowing and warpage.
If it's petroleum based - rubber, plastic, etc - it's dead or 80% of the way to the grave already. You just haven't noticed. Even the hoses that "look OK" aren't. They are shrunken and inflexible and will crack and peel and leave your hands BLACK if you get within the same zip code.
