[RETed]

First of all, is the gear oil you got "GL-4" rated?
GL-4 is specific for (manual) transmissions with brass synchronizers.
There is gear oil that are "GL-5" rated that is not designed for our transmissions.
The GL-5 does not supercede the GL-4 rating, except in load bearing testing.
Sulfates are typically used to increase load rating, but the same sulfates can eat the transmission brass synchronizers.
Please read the gear oil bottle labels to confirm this.

Typically, the shifter extension housing is empty from the factory; Mazda does not fill them up with gear oil.
There are two other sections in front of the shifter extension housing that house most of the transmission gear and internals that need the trans fluid.
The sections are only connected by small passages at the top of the transmission.
This means to get trans fluid from the shifter extension housing to both gear sections, you need to fill all of them with trans fluid.
This will use slightly more than the factory spec 2.6 liters(?) of fluid.
(We - in the U.S. - typically get this stuff in quart bottles, and you need at least 3 quart bottles anyways, so this isn't that big of a deal.)
Here's the more immediate problem...
Since everything is now filled, your shifter boots better be in good shape, or else it leaks out from the shifter lever.
This is why Mazda doesn't fill the rear, shifter extension housing in the first place.

Although this is usually harder and messier, I'd recommend using a mechanics syringe to fill the trans from underneath.
Just fill the front two transmission sections as described in the FSM.


-Ted