synthetic or conventional

You must use a GL-4 oil. I highly recommend Redline Synthetic http://www.redlineoil.com/product.aspx?pid=46&pcid=7
You can get it shipped to you by Amazon or Summit. I think I used 4 or 5 quarts for my trans. Makes a big difference in the smoothness of shifting and is not as sticky in cold weather.

Or save some money and get the same MT90 fluid from ASCO. http://store.acadianasportscars.com/asco503043p

I bought a 3-pack from them last month. Used two quarts in my T5 and have one left for the differential. Your 71C should use the same amount.

Ummm. I remember reading a pretty old post about GL4 vs GL5 oils. There was a recommendation for Motul Gear 300, which I am currently using… but now I am googling it and a few results pop up saying the Motul Gear 300 is GL4/GL5 (i.e. not good for GL4-only applications)… did something change?

Edit: This thread: https://z31performance.com/forum/z31…-can-be-a-pain

Edit #2: More digging… from the s2000 world:

http://www.s2ki.com/s2000/topic/9291…y-experiences/
http://www.s2ki.com/s2000/topic/9482…-differential/

You need 3 qts for the 71c and youre supposed to use ATF in the T5.

Thank you all !!!!!!

If it takes more than three something is very wrong
As much as I love synthetic I'd never use it in my CLSD diff.

Interesting… Amsoil's spec sheet shows 4 pints for the 71C in the Z31. I don't have one to compare to, so my mistake if incorrect. A 3-pack is still sufficient for the transmission alone then.

However, ATF is recommended for the WC T5, not the Non-WC T5 that came in our Z31s. WC T5s came with fiber-lined brass synchros and require ATF to keep the lining intact. Non-WC T5s have standard brass synchros and need a GL-4 type fluid. Run ATF in a Z31's T5 and you'll find that first and second gear synchros can develop a nasty whine.

There's a sticky on this linked above. I would recommend checking it out.
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Currently running an open diff.

With that said, I've used synthetic in other CLSDs before. Most brands recommend a limited slip additive to keep things quiet and happy.

The transmission is supposed to be filled to the top of the fill hole. I dumped mine, plugged it, and it took about 3 quarts of mt-90, there was a small amount left over.

I have heard that as well, the WC was the only one to take ATF, but I have heard the argument that the non WC can take it as well. I don't know who to believe, and I honestly think nobody knows what they're talking about and its all opinion. I've read about people swearing up and down atf goes in ALL T5's, and others say 75w90 works fine in the non WC too, including the service manual, but there's also some outdated shit in some service manuals.

I think it was on bobistheoilguy, multiple pages of people bickering over the WC and non WC T5, and whether to use ATF or 75w90.

I can dig out the owner's manual on my 86t and show you where it claims to take either 80w/90 or dex.

Having tried them both I'm a big fan of Redline D4 ATF.

Never thought I'd say it, but the expensive stuff makes a diffence over conventional ATF.

Manual transmissions and all differentials are designed to be fillet to the bottom of the fill hole NOT THE TOP

just a note for anyone reading this. this guy is 110% wrong. only listen to him if you like blowing up your differential and transmission.

No, actually he is right. You fill until fluid starts coming out of the fill hole. How the heck are you supposed to fill the case to the top of the fill hole when it is pouring out of said hole?

Lift one side of the car higher while filling

At least with the transmission, the FSM also specifically says to fill up to the hole, not higher in any way.