Simple ?? What causes the most or least wear on combines?

Corn?
Beans? 
Wheat?
Oats?
I do mostly corn and beans on a small farm with some oats too. I don't mind buying a higher hour combine if it as been well maintained or perhaps used in a lower wear crop like what I think oats might be? Would a 3000 Sep. HR machine from Canada be likely to show more or less wear then a mostly bean combine from the South?

Thanks,