I use all castor but I can't really say what the life of a bearing is because I've never had one fail. Then again I've never bothered keeping track of how much running any of my engines have had except my ST G51 which has had ~350 flights (30 hours to be more precise). An old Enya 45 of mine did somewhere in the hundreds of hours (and wore out one set of rings) but the bearings were still perfect.

Oh, and I've just got my very first brand new ABN engine, an OS 46VF , so soon I'll be starting the break in. My way....