I too used Castrol GTX 20w/50 in the (engine &) gearbox for decades, but when I had to succumb to buying a *store bought* overdrive unit for my side-shifter xmsn__I didn't have the NLA parts laying around to fix it__I turned to Quantum Mechanics for a reconditioned unit.
As per John @ QM, if I didn't use 30w non-detergent oil, the warranty was void. So nowadays, the trans has 30w non-detergent (Shell?) oil in it, along with a couple bottles on the shelf. To put everyone's mind at ease, it too is seemingly self-replenishing, confirmed by the oil-soaked piece of cardboard under the car, at the dipstick still at the MAX line...
... how it does it, we'll never know