There are two basic challenges to changing the transmission fluid on the E30 for the DIY’er. The first is to find a 17mm hex (a shallow 17mm at that) to undo the transmission fill and drain plugs. The second challenge is how to actually get new fluid back in because the drain plug is surrounded by the transmission tunnel, some people try to make contraptions that allow them to pump the fluid back in; my method uses good ol’ gravity.
Things you will need for this job:
17mm bolt (the head must be 17mm)
A nut that will fit on that bolt
Basic small funnel
You will use the bolt and nut as a tool to undo the hex drain bolts. The 17mm head of the bolt will fit in perfectly to do that job. You will put your ratchet/wrench on the nut end. The only hurdle is finding a way to secure the bolt to the nut, because you will be turning the nut counter clockwise (which would make it come off the bolt if not secured). I had a welder handy and just welded the nut and the bolt together, however if you do not have a welder, you can try a few things:
1. Use permanent strength thread locker (Loctite red), this should have enough holding power to allow you to undo the plug.
2. Drill a hole through the bolt after the nut and safety wire it.
3. Buy 2 nuts and tighten them into each other extremely tight, then put the wrench on the upper nut
4. Buy a 17mm hex if you so choose
To fill the transmission:
Take your small funnel to your local hardware store and go to the plumbing section and find the appropriate clear tubing diameter where you can fit the end of the funnel into it (like a beer bong). This tubing is something like 50 cents a foot, and you buy it by the foot off of a big roll, 4 feet should be enough (2 bucks). Proceed to feed that tubing though the inside of your car, through the shifter tunnel, just move the rubber boot to the side and then go under the car and feed it the rest of the way into the transmission fill hole.
Then proceed to fill the transmission!
Check your transmission for any stickers/labels:
It is critical that you use GL-4 for the transmission, do not use GL-5.
Manual Transmission fluids
Green Label: MT-90 (75W90 GL-4)
Red Label: D4 ATF
No Label: MTL (70W80 GL-4)
Since this method can also be used for differential, sans having to make the bolt tool since the the drain plugs are much smaller:
75W90 GL-5 (Suitable for regular and LSD units.)
75W90 NS GL-5 (Suitable for regular units only.)