Have 18,750 service scheduled with dealer. Already had cabin and engine air filters replaced at 15,000 service. Should anything else be checked?

 

http://www.mazda.ca/files/2009_rx-8_om_eng.pdf

pg 335

Depending on how conservetively you drive, of course....

I'd probably recommend taking a look at your plugs, but if your dealer's doin all your maintainance and you know and trust them then you should be good.