I have Royal Purple SyncroMax in my 15 since about 2.5k miles, have 21k now.
It made a big difference in the smoothness of the shifting. Still a little notchy going into 2nd when its cold (below 30F) until the tranny warms up.
With this product, do you drain and replace all the gear oil, or is this used as an additive?
I'm guessing it's a replacement for all the gear oil, I just want to be clear. One of my 2014 GT's could use some improvement, the other is slick as snot.