I was wondering if anyone here has tried overfilling their manual transmission by about 1/2 a cup worth of fluid to try to smooth out shifting on a G35 or 350Z(this should not be enough to cause frothing in the oil, I would think)? After searching a bit on several Z and G forums, I wasn't able to locate anything about anyone else trying this trick on their G or Z.

I am still running the stock(from the factory) fluid in my transmission, but I think I'll probably do a change in the next 3K miles or so when I get close to 20K on the car to the Shell spyromax(sp?) gear oil if I can get my hands on some, or possibly Motul gear oil.

As far as overfilling is concerned, I've done this before on my Tacoma and my 240SX and noticed that it seemed to smooth out shifting a bit, particularly when the vehicle is cold. I tested it by filling the tranny..... driving a bit, waiting about an hour, adding an extra 1/4-1/2 cup of fluid and then driving again. The easiest way to overfill is to fill the car with it parked on level ground.. put your filler plug back in, then drive one side of the car up onto 2x4's or really low ramps so that the side of the vehicle housing the tranny filler plug is slightly elevated above the non-filler side of the vehicle(thus allowing you to add a bit more fluid to the tranny before it overflows out the filler hole). Or you could remove the shift lever from inside the car and add a bit more that way.

What gave me this idea is that I've noticed that a lot of manual transmissions on various different cars come slightly overfilled from the factory as evidenced by the fact that some oil would flow out of the filler hole when the filler plug is removed with the car parked on level ground and the vehicle being completely cold(meaning that the gear oil in the tranny had not expanded due to being warmed up to operating temperature). I could see how this might happen in a 4x4 with possibly fluid transfer between the transfer case and Transmission resulting in elevated or slightly lower levels in one or the other... but obviously this can't happen in a RWD car such as ours which aren't mated to a transfer case(excluding the G35X obviously).

So I guess I really have 2 questions:

1) Has anyone tried overfilling their manual transmission to see if it improves shifting smoothness? If so.. what were the results?

2) Has anyone tried removing the filler plug on their manual transmision while the vehicle was cold and noticed if any fluid seeped out of the filler hole while the car was parked on level ground(Indicating that the tranny was slightly overfilled from the factory)?