I grew up in an era where 10k on plugs was good. Cleaning up the way engines burn have increased the lives of all the components. I still like to look at the plugs in a motor every 30k at least just to verify whats going on. Oil consumption/carbon, mixture history, alot can be read.
Your motor does 30k just fine on Bosch F8DC4; a rather inexpensive plug. Gotta be $2 or less at the discount store.
Bosch Master, ASE Master, L1
33 years MB technician