synthetic or conventional

I just changed gear oils in my T5 (FS5R90A) and the rear differential (Quaife). In the transmission I changed from ATF to Liqui Moly GL-3+ and in the rear to Liqui Moly GL-5. Right in front of me is the 1985 OEM Nissan 300ZX FSM and on page GI-13 it specifically states that in the T5 Turbo model one can use either GL-4 or Type DEXRON, which is an ATF. For some reason, non-turbo models cannot use ATF. Liqui Moly GL-3+ is usable for GL-3 and GL-4 applications and is made to work with brass syncros. GL-5 is too slippery for the T5 brass syncros --- and believe me, it is the slipperiest oil I have ever felt.

This is likely because the non-turbo models did not use the T-5.

i meant to say to the top hole. my bad, but seriously how the fuck wasn't it obvious what i meant. i thought the other guy was saying to NOT fill it fully.

@OP. Mobil 1 Synthetic Gear Lubricant LS 75w-90 is an API GL-5 product which is intended for extreme pressure protection in differentials and not transmissions. The sulfur/phosphorous content is too high and will corrode the brass synchros in our transmissions.

OK, so I have seen a lot of people asking on z31performance.com and several other forums, "What type of transmission do I use for the to use for the 71C?" and "What type of transmission fluid do I use for the T5." Many folks have recommended synthetic, or ATF, or 50wt, or Redline MT-90 because they say it, "made the car shift smoother." However, I've discovered that some folks really don't know why one type of fluid works better than another. Perhaps they think because it's better because it got good reviews. Some folks heard it through word of mouth. Or maybe it's because one costs more than another, therefore it must be of a higher quality. In order to clear up all of the confusion, I'm posting a short summary of exactly what type of gear oil should be used in the 300ZX non-turbo FS5W71C, 300ZX Turbo FS5R90A (Non-World Class T-5), World Class T-5, and God forbid you have one, the automatic transmissions.

E4N71B Automatic transmission: ATF DEXRON

4N71B Automatic transmission: ATF DEXRON

FS5W71C 5-speed transmission: Nissan recommends API GL-4 (SAE 75w-90)

Non-World Class T5: API GL-4 (SAE 80w-90) or Type DEXRON. The FSM is outdated and things have changed since the Z31 made its debut. I could go into details about DEXRON, but to avoid confusion look for DEXRON III which is still sold, or ATF labeled "Dex/Merc." Buy a trusted brand such as Valvoline.

80w-90 is what I use in my T5 transmission, and it has many additives that provide cushioning for the gears and syncros. It works perfectly fine in the spring and summer months. However, it's thicker in cold weather which can result in slower shifts. That's why some folks recommend using ATF. ATF 10w is lighter so you'll get quicker shifts in cold weather. Synthetic gear oil has the same viscosity as conventional gear oil, but a higher flow rate. Therefore, it will also provide quicker shifts in cold weather. However, I don't drive in the winter months and the car is over 30 years old. In this case, conventional 80w-90 is fine for me. Saves on the wallet. If you drive your Z in cold weather, by all means use ATF or synthetic.

As I stated, DEXRON III or ATF Dex/Merc is recommended for the NWC T-5. but not API GL-5. API GL-5 contains twice as much sulfur/phosphorous as API GL-4 which is corrosive to the brass synchronizers. That will cause your synchronizers to wear out extremely fast. API-GL 5 is needed for extreme pressure protection in the differential where the final torque is applied. The higher sulfur/phosphorous content helps it bond to the brass. API GL-4 is used in the transmission where the extreme pressure protection isn't needed. Only 30% of the torque is applied in the transmission.

World Class T-5: ****ATF DEXRON III****. The World Class T5 came with a sticker on it that said, "ATF Only." They don't use heavy gear oil or Redline MTL.

I would change that out immediately. Motul 300 is great stuff (used it in my WRX), but apparently will breakdown the copper synchros. I've been using MT90 for a while now and never had any problems. I'm no scientist though.