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)?

 

My tranny was definetly overfilled from the factory (got a new tranny from Nissan w00t). I'm going to start changing my tranny fluid every 10k miles since I'm racing the car so tell me what your findings are. I'm all for more fluid shifting.

 

Most seasoned mechanics will highly advise against overfilling differentials or manual transmissions. We usually fill them up to the brim, and then let a steady stream drip out, before capping them off.

 

Absolutely, I agree. I used to work for Nissan in Bloomfield, NJ and that's what most of us did... But to answer your 2nd question, yes, most come overfilled from the factory. That's due in part cause the engine and trans are mounted together prior to putting into the car. They assemble them, then mount the engine to the tranny, then to the car. Assembly line...

 

How does this result in them being overfilled? Assuming they are filled on a level plain, the excess fluid should still flow out of the open filler hole when it's full, unless they are filling it from above through the shifter passageway with the filler hole closed and just going by look(or measurement) as to how much to dump into the tranny.

 

Not sure....I havent noticed them overfilled from the factory. But they may tilt the tranny when filling, as I assume it would be easier to automate this process, vs. filling from the side. No clue really.

My guess is that they fill it to a specific volume, rather than site.

 

Is it necessary to change manual transmission gear oil? I was going to get mine changed at nissan and they said it wasn't necessary. I've got 44K miles on my 2003 6-speed and have not changed it.

 

I dont have the factory service intervals in front of me, but the fluid does not need to be changed for at least 60,000miles, IIRC. Of course, the performance guys are usually changing to high-peformance synthetic or race fluids more frequently than that.

 

My current tranny had about 25K before I changed the tranny oil. I started getting a bad grind in 2nd every now and then. Changed to Redline MTL/MT-90 mixed and going into second is a lot better.

I'm sure you're okay if you wait until the interval but can't hurt to change it now

 

I changed my tranny and diff fluid at 17k miles, and I'm glad I did. The diff. fluid was gray/black with black powerdy shavings in it, and while the tranny wasn't anywhere near as bad, I could see tini coper shavings.

Tranny fluid:
https://my350z.com/forum/attachment....9&d=1135218513

Diff fluid:
https://my350z.com/forum/attachment....8&d=1135218513

Old vs New:
https://my350z.com/forum/attachment....1&d=1135218513

 

I just changed both tranny and diff fluids...

the diff fluid was horrible, it was murky brown and smelled heavily. The magnetic drain plug had quite a bit of metal dust attached to it.

the tranny fluid was still very healthy golden brown, and did not smell. my tranny was NOT overfilled, when I unbolted the fill plug not even a drop came out. I could not notice any copper shavings... (BTW, this is a 03 original tranny, with no grind, lol).

Changed both tranny and diff to RP 75w90, haven't had time to drive the car yet so can't give impressions yet.

 

Nano...

How many miles on your car when you changed out the diff fluid and noticed that it was revoltingly dirty?

 

not many, only 25K miles (I track the car a bit though )

 

Did you install a diff cooler and if so what kind. I hear track driven z's need a diff cooler to stay effective.

your thoughts ?

 

Weird... It's been almost 36 hours and this guy hasn't responded.
He only just finished crafting his response several years ago, and he hasn't been online in a few years, yet there is still this awkward delay.

 

nice...