I am trying to check my fluid in my 5 speed in my 90 240sx. I cannot get the damn fill plug out. It is square shaped, but a 17mm open end wrench will fit over it. I've tried even using a pipe for leverage on the wrench. I put so much force on the plug that eventually the jaws of the wrench flex before the plug budges. How can I get it outAlso, what type of gear oil should I use to top off the tranny? My chilton's manual only mentions using an API GL-4 rated oil, but fails to mention which viscosity? The reason why I suspect the fluid is low is because there are two electrical sensors stcking out of the passenger side of the main tranny case and they are seeping gear oil. How do I keep these sensors or whatever they are from seeping fluid? They aren't near the bottom of the case so they wouldn't drain the oil dry from the sump, but they are at a point where they could significantly lower the fluid level.

You should actually be able to put a 1/2" drive socket wrench into the plug and turn it with the wrench. As to what weight of gear oil, my service manual suggest 75W-90 gear oil for the transmission and 80W-90 for the differential.

I know the drain plug can be removed by a socket wrench because it is a female square shaped hole. But I am referring to the fill plug. The fill plug, which is up higher on the tranny has a male square shaped head on it, which will accept a 17mm box end wrench. That's what my question is, how do I get that out??

You need to get an 11/16 square drive socket , it is an 8 point socket that will fit it correctly.

the oil needed is 85W90 , or if you live in a colder climate you can use a 50/50 mix of ATF and 85W90 gear oil

get under the car, stick your legs on the bottom of the bumper, wrap an adjustable monkey wrench around the bolt and giver hell. That's what I did and it worked.

Ok for the filler, i would rather just pull the shifter out and get a funnel and go in from the top. Of course i have a C's short shifter that is a BREEZE to install and its all very easy and theres no mess trying to squeeze the fluid into the transmission sideways while lying on my back under the car. Sorry for the misinformation.