go down to BNT, ask for Redline gearbox oil
cost you $27 i think a litre
youll need 3 litres (2.5L litre box), a size 24 SINGLE hex Socket, and a half inch strong arm bar (if you have the room to leverage it) other wise use a half inch socked (provided the socket IS half an inch lol)
first undo the Filler hole bolt, so its loose and easy to turn via hand (also allowing air in), second, undo the bottom drain one, making sure you have a catch bucket / can thing below to catch all shit.
Undo the bottom bolt, let it all run out, as soon as you have undone the bottom, undo the top to allow it all to flow out.
let it drain.
if you really want, run bit of clean fluid through via the fill hole to pickup any shit while it runs out again, this isnt really a big deal though.
righteo, now do up the lower *drain* plug. nice and tight, but dont round the bolt. Is there a torque measurement on these people?
grab your squirter thing, (i forget what they are called....) and Oil Greese Gun i guess, but its not a gun! lol, anyway, grab a slob of oil in the gun, and chuck it into your gearbox, continue until oil flows back out the filler hole, then chuck the bolt back on. again, nice and tight, but not TOO tight

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sort of oil sump plug tight i guess is a way to describe it.
do up everything you took off to get to it (my ae111 had a plastic cover thing, that i took off to get to the filler plug).
Jump in the car, crank her up, give it a drive. I doubt youd feel it instantly, my 111 just felt the same until a few days later, its much better now, but as soon as i done it, i thought i had wasted my money for sure

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Peace of mind though, Peace of mind.