I have found the TAO3 ball diff to work in the MO3, MO4, MO5 and MO6. Just in the rear wheel cars I haven't tightened it like I do for my FWD cars.

To me the MO5 ball diff is of higher maintenance if you want to crank it tight as you have to use different Tamiya bits, so that it stays tight. If built correctly these work very well, especially with the hardened diff outdrives from the MO5Ra.

The 3Racing ball diff can also be made to work well and be durable.

Some others prefer the gear diff with a couple of extra washers and or special building techniques which make these tight. The new hardened gear diff being released mid January and that comes with the MO6 Pro is a bit tighter when built than the old white gear diff. This will come with hardened gearbox gears. I am currently using in a MO5 that I have started building.

For servo's, I have previously used Hitec in my mini, but found them toc ause me problems and have since moved to Futaba digital servo's which I now prefer, so I agree with sosidge on this point. On my Mo5 Pro and MO6 Pro I have used the provided Tamiya servo saver with no problems. If I had issues I would use a small Kimborough servo saver.

Just my opinion.

Later,

Calvin.