Hard to turn shower hot and cold Kolher 1/4 turn valve stems

In wall of my shower have a kolher Triton K-T7751-2-cp trim set using what i believe to be according to Kohler website a valve set of k-303-k.

The installation is probably a least 20 years old. The stems are 1/4 design and have always been hard to turn but with no leaks.

Can they be adjusted, greased, seals replaced etc?

They are not serviceable. Replace the stem units.