Here's a little more pictorial explanation. I've attached a couple photos that should help understand it better. AJsToyz has a pretty good explanation too. Basically there is more friction and heat associated with the bushed bearing since it is just two pieces of metal rubbing against each other hence the necessity for more oil in the fuel. Realistically it is much more than just two pieces rubbing against each other but that's a much more detailed answer. The needle bearing provides a much "smoother" ride than the bushing but in sizes this small it is difficult to make a bearing strong enough to handle the forces that engine operation places on the bearing. The bearing needs less oil content because it is not generating as much friction and heat.

I hope that wasn't too involved but still helpful.

Jimmy<br type="_moz" />