There are two plugs on the 300zx manual transmission. on on the passenger side and one on the bottom of the transmission. You can unscrew them with a 1/2 drive ratched. fill through the plug on the side, drain from the plug on the bottom
The FSM calls for a 'Dexron' type transmission fluid.
Yes my car is a 1990 300zx with a 2008 350z motor and 6speed transmission
Automatic Transmission fluid
To add transmission fluid to a 1990 Nissan 300ZX, first, ensure the vehicle is on a level surface and the engine is running. Locate the transmission dipstick, usually found near the back of the engine bay, and remove it to check the fluid level. If it’s low, use a funnel to carefully add the appropriate type of transmission fluid through the dipstick tube until it reaches the recommended level. Always check the owner's manual for the correct fluid type and specifications.
75W-90 GL-4. Redline MT-90 is the best but, you can use whatever you want
In the transmission.
You put the Transmission Fluid in the Transmission (fill point transmission drip stick tube)
To change the transmission fluid in a 1984 Nissan 300ZX, first, warm up the engine slightly and then turn it off. Locate the transmission fluid pan, remove the drain plug to drain the old fluid into a container, and then replace the plug. Next, remove the pan to replace the filter if applicable, clean the pan, and reinstall it. Finally, refill the transmission with the appropriate type and amount of new fluid through the dipstick tube, checking the level as you go.
Into the power steering reservoir under the hood.
The power steering fluid reservoir is up on the passenger side wheel well in the engine bay. Looks like a short fat silver coke can with hoses coming out of it. On a side note, the 300zx twin turbo uses ATF (automatic transmission fluid) not power steering fluid. usually a green casing with black top. the following link is NOT a 300zx, but the reservoir is about the same. http://www.sillbeer.com/images/engine/power-steering-reservoir.jpg
The transmission will fail.
Low Transmission fluid