MS4 and dual spring maintenence
If you want you could always do them as part of a service at a set interval, but I don't really think you state that it must be this milage or else kind of thing.
If you beat on the car all the time in the high rpms and/or use a decent shot of nitrous the wear is going to be increased. If you spend 80%+ of those 20k miles babying it in town or at 50mph on the freeway then they will last a lot longer.
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