I run my SC6 in mid motor configuration and use a Schelle Aluminum Top Shaft, 2x Kyosho Aluminum Idler Gear (KYOUMW718), and the MIP Plastic Differential Gear. Yes, the Kyosho instructions do not recommend this configuration, but I was tired of stripping the plastic Kyosho SP Ider Gear (KYOUMW513). Kyosho should have put stronger plastic idler gears in these 6 series cars instead of using parts from the previous generation of cars. That is my one beef with Kyosho and how they always do this. Anyways, I put black grease all over the gears to provide lubrication. I have had NO issues running my SC6 this way for the past 6 months. I race quite a bit and run an 8.0T motor in my truck on a high bite track. I rebuilt my differential last weekend and examined all the gears. There was no noticeable wear on the gears. Save yourself money and go full aluminum idler gears. If/when the aluminum ones wear out, you go buy two new ones. Simple as that. Personally, I don't have the patience or time to deal with this design flaw. Further to this, I also saw pics that Jared Tebo does the same idler gear setup as me.

I still carry a completely assembled spare transmission (with 2x aluminum idler gears) in case of emergencies (habit from stripping so many plastic idler gears). I would recommend my setup as long as you make sure the gears in the transmission have some sort of lubrication (black grease). Not sure how you guys are getting dirt inside your transmissions cases. My track is an indoor track so there is less dust.