Here is a link for a PDF on GL-4 vs GL-5 info. We recommend everyone installing a new transmission/transfercase read and know this. It is a good informative read on the difference on the oil types written by Richard Widman. He covers everything you need to know and should know, it is a very extensive comparison of the oil types.
-Transmission/Transfer-case guidelines and tips to keep in mind when purchasing and installing-
If the mainshaft of a transmission is locked in place by a wire or plate, do not remove this untill it is installed or right as your installing it. The tie wire or plate is there so the mainshaft does not fall out of place, if that happens it can cause big problems.
New synchro rings inside a transmission can get stuck or jammed onto the gears in transport, this can cause difficulty shifting. Simply spreading the synchro ring from the gear(s) with a flathead can free them up.
If you need to Install a yoke or Flange, use RTV silicone on the splines under the washer before tightening the nut down, this will prevent leaking.
Make sure any soft plugs or sensors in your shift tower are present and secure to prevent leaking.
If you are installing a T-84 make sure the Interlock pin between the transmission and Transfer-case is installed correctly or it will not shift correctly and may cause damage.
Before bolting gearboxes together make sure you clean the gasket surfaces. Use a thin layer of RTV silicone and the correct gaskets, also make sure all the bolts you use are the correct length, if they are too long this can cause problems with operation of the gearbox.
New gears may be a little noisy. Changing the oil after 300 miles or so can help the break in process, and will most likely quiet down your gearbox as well as help the overall life of it.
Most of our gearboxes including the T-84, T-90, and D-18 use 80-90W non-synthetic oil. GL-4 is ideal, GL-5 also will work though. If you have a different gear box make sure you are using the correct oil for it.
