I believe the drain & fill plugs on the side of the transmission takes a 10 mm allen wrench .
Tester: All of my British cars used 30 weight engine oil in the gearbox and lever action shock absorbers.
I don’t buy “never change manual trans oil” and here’s why:
Gears and synchros shed metal particles even when everything is working properly, especially early in the transmissions life.
Bearings don’t “like” metal particles swirling through them.
Clean, fresh oil is cheap insurance.
My Corolla uses a similar style of hex key (allen wrench) manual transmission fill plug. I didn’t know what size of hex key to use either, and the factory service manual didn’t say, so I just cleaned the gunk off the area and measured the size of the plug myself. Measure the distance in mm between opposing sides of the hexagon with a set of calipers and a ruler. On the Corolla with the M5 manual as I recall the key size is somewhat bigger than 10 mm, but can’t recall now exactly what it was. But it was a simple enough thing to measure.