I'm not sure what the factory put in mine. The oil smelled like gear oil, but was very thin. The Datsun manuals don't say anything about T5 maintenance though, they seem to have forgotten to include it. I suspect Nissan shipped theirs with 75/90 weight. So I called the Pennzoil 800# number from their website, and also spoke to the local T5 guru who fixed mine. Both said that the trans specs call for 75/90 gear oil or ATF.

* 75/90 has a lot of additives and cushions the gears and synchros well, but gets thick in cold temperatures.
* ATF (10W) is used to give quicker shifting and gear engagement, and makes cold weather shifting easier.

This T5 lore may be why some people use ATF in normal Z transmissions. But I don't think that's wise: 1970's transmissions are designed for a heavier oil to cushion the brass synchros and gears. The T5 can use ATF because it was designed to, not because the oil is superior.

Dexron-3/Mercon ATF came out in 1996. Before that there was Dexron 2 and before that Dexron 1. As they evolved the detergent and lubricating properties greatly increased over the years, making ATF an excellent oil. There are several ATF versions though, make sure you don't get "Type F" or "ATF+3". "Dexron-3/Mercon" is what you want. All brands have to meet the same standards to say Dexron3/Mercon, so the brand is up to you. I use synthetic-blend Valvoline ATF.

A Ford Muscle Web article claims 75/90W ruins world class T5s. This is because T5 oil passages and bearing clearances supposedly are smaller than regular transmissions, and a heavier gear oil can have trouble flowing through.

Oil capacity 2.55 litres.
Standard oil grade ATF.

One last piece of important information. ALL World Class T-5 use ATF oil and NOT heavy gear oil or Redline MTL! All NON-World Class T-5 use 70 wt gear oil. We recommend high quality Synthetic oil as it lasts a long time and over a larger operating temperature. It is important to change the oil in your transmission based on how you use the vehicle. If the vehicle is your daily driver then changing the oil every two to three years is fine. However, if the transmission sees regular track duty and pushed hard, then once or twice a year is in order. When we rebuild transmissions, we can tell when the oil has been changed often as the parts are much cleaner with less over all wear. There is no filter inside the manual transmission only a magnet at the bottom to pick up the metallic particles in the oil, the only metallic parts in a transmission is the gears or bearings. Oil is cheap, transmissions are not!