Gear box thrust washer

If it's  the Delrin plastic washer you're refering to it may have gotten desinegrate.  It may have drained partially out when pulling the gearbox of and some small particles flushed through the system.  I doubt hat was left out by the factory.

Thanks Roger, I cut the filter and found nothing there. Wondering if I need to be concerned about debris stopping up oil passages in the engine.  

Hi Jim

As Roger said the washer may have been ground up.  If there is no trace in the bottom of the oil tank then it may have passed into your oil cooler and if it made it past the oil pump the filter element will trap the small debris.  In your oil change you would only see tiny black spots on the filter element.  There is little to no chance it will get into any oil passages as any parts such as this are directly flushed into the oil return galley in the crankcase and pushed directly to the oil tank after that.  

Cheers

If oil pressure or temps starting doing something odd then maybe an oil cooler flush which is easy to do. Just keep and eye on things for a while. Nothing may happen at all.