Besides tires, transmission service is such an annoying service you need to do all the time. What a headache after you build a new trans, you run it a few races, and it is gritty again! The team has tried countless ball types in steel, stainless steel, ceramic, etc. from a variety of suppliers with the same result: few race events and the gritty feel returns. Then every diff ring was then tried, thinking the ring was causing the balls to flat spot, etc. and with the grease problem, here is where that gritty feel comes from. The diff ring experiment did not resolve the problem either.
One of the consistent things we see inside the transmission after a few runs: where did the clear grease on the diff balls go? For a while, we used large diff gear with square holes and forced grease into the area in the square holes thinking the grease would stay there longer. No such luck again! Then tried every manufacturer of the large diff gear thinking there must be a better idea, but the grease migrates where it is not needed. For quite a while, we just gave up, resigned to the fact that gritty feel will return fast, and always have a spare trans available to slide in at the track.
We did not stop trying to find that prefect grease. By chance one of the suppliers I knew from many years ago (not RC related) suggested a manufacturer which had a grease that sounded like what is needed, and I sourced a sample for the team to try.
What a difference a grease can make! After a few runs, the trans still did not feel gritty. Opened up the trans, and much of the grease was still stuck to the diff balls. We tried it will a variety of diff balls, steel and ceramic, and the grease hung on to those diff balls better than anything else we have used. This grease clings much better than before.
After more use, we think this is the grease for best life in any transmission. Nothing is permanent, but this grease just feels stickier than the other greases and seems to cling better to those diff balls. Until something better is found, this is the grease to use. Try some!