Must rear diff and MT fluid weight match?
 

Starting to develop a 4th gear grind from 3rd to 4th, 5th to 4th is fine. Only happens after the car is fully warmed up. I believe a more viscous MT fluid will rid me of this nuisance, however, I don't want to swap the rear diff fluid to a thicker fluid. Is it recommended or required that those two fluids match in weight?

The MT gear fluid is independent of the Diff gear fluid. The two are not supposed to be the same.

Rear diff fluid is GL-5 and will ruin your synchro's if used in the manual tranny. You need GL-4 fluid for the manual tranny.

The owner's manual specifies different fluids. Why would you think they need to match?

Weights of fluid, not types. I understand that there's different requirements for both.

Possibly a snychro issue for that gear?

The weights do not have to match. Since they do similar jobs (torque multiplication via gears) under similar loads, the spec'ed weights can be the same ... but they don't have to be.

There are several threads about changing to different trans fluid to help with shifting problems.

Let's start here, have you changed any fluids in the car or had them changed recently? Or are you just starting to develop a grind? To each their own but Nissan's mtf is pretty damn 75w-85 is the weight you should stick with. The grind is independent of your rear differential. So changing the diff fluid won't help you. When was the last time you had the clutch fluid replaced?

You can put any weight in the tranny. Long as it's GL-4. I've see trannies run from ATF to 250w. But remember. The heavier the weight. The harder the shifting. In cold weather. It will be stiffer.

:iagree:

^^^^^

What he said

Moving beyond the orginal question. Nissan Synchros in the Z are not all that great for what ever reason, many people have issues with it, a few so bad that they had them replaced under warranty. Personally 4th to 5th is the worst for me. I have to shift slowly to avoid grinding. Odd thing is it never gives me any issue when I am at the track shifting at near red line :ugh2:.I have changed the gear fluid in the MT to a 50/50 mix of Redline MTL and MT-90, which is now MT-85. I did this to have the transmission shifting feel smoother, it had no effect on the synchros.