STOP PRESS! Via the power of Google I've managed to find out (duh!) just exactly what a hypoid diff is all about; namely, the centreline of the pinion shaft does not pass through the c/line of the crownwheel.
Developed for front engined-rear wheel drive cars so the floor line could be lowered a bit.
Cross-section drawing of the 2CV final drive confirms that the pinion line is through the crown wheel c/line, and probably wouldn't need to be any other way in a front engine-frontwheel drive car(?)
So: for the Duck, there's no need at all to use a hypoid rated oil - but ... are the synchro rings "yellow"? - over to you, Ken.

because the Haynes manual is sponsored by Duckhams, the early ones by Castrol

The oils and lubricants section quotes the suitable product from the sponsors range- even if that was a hypoid product ?

the fact that the crown wheel and pinion arent technically a "Hypoid" doesnt mean that they wont benifit from the properties of a hypoid oil there is still an element of shear involved in the action of the gear

Sean

Some of the issues related to A model gear boxes is.

1. Oil changes are not done at regular intervals. Less protection more wear.
2. No service, maintenance or using low quality parts. Nearly all gear boxes are way off original Citroën setting even after a few years of driving.
3. Incorrect driving, common belief is to hammer the shit out of car. Our right wrong. Imagine the force put through the gear box, especially when older gear boxes has play virtually throughout the entire box. Gear teeth are hammered again each other. NO oil can ease this.

Still a reliable A model box will haul and scream at you for years before it finally breaks, not bad I think.

i dont think that many 'boxes were that well set up in the first place. - close enough was good enough!
put a 40k Portugese box on mine last year to take my good one in for some tlc, and the lash is shocking on that low mileage one. my screamer was tighter despite having a worn input and upper rear bearing.

Sean

Hello - he says sheepishly....

I'm getting very confused. I'm not at the gearbox knowledge level yet....

My gearbox is a drummer, with the separate rectangular tin-top plate.....

What gearbox oil should I use??? would rather take advice on hear....so cheeky of Haynes / Duckhams - but that commerce i guess

Cheers all
Will

Just go down halfrauds or other motor factor and get some EP75/80 or if you like it thicker EP80/90 . seeemples

Thanks Sean - but I'm really really green on this:-

is my gear box

1. mild extreme pressure?
2. extreme pressure?
3. okay with GL-4?
4. okay with GL-5?
5. Hypoid?
6. not Hypoid?

imagine i've just dropped from the sky with shit for brains

Thanks

and your me..

this still confuses me

id love someone to say get this product <picture> from here <name>

simples

What part of go up halfrauds and get some EP75/80 or EP80/90 is hard to get They (and others) do castrol and own brand and its all fine..

The trouble is, sometimes people over complicate things for themselves by looking too hard into everything ,i'm not saying put sunflower oil in but its a 2cv ffs ,no more complicated than a 1940s bmw motorcycle and it takes EP 75/80 or EP80/90 in the gearbox. Easy!!