Your figures are correct.?? That is, drain and refill is to be 235 cc (265 is obsolete); and, for overhaul (that means dry to start the refilling), 25o, as 280 is obsolete.
But, be advised that BMW was installing a red oil back then, and its viscosity was not specified in the usual manner you may be used-to now-a-days.? Back then, the actual viscosity was closer to 4 SAE, and in some parts of the spec, one could say it was 7.5.? Yeah, confusing.

What is not confusing is the oil itself.? Here in the USA, the oil being used, and for refills, was military hydraulic fluid, Mil 5606-.? It is widely available.
Use of a 10 weight fork or suspension fluid would work fine.
For an article:??

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Snowbum

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Donald,

You can make your own 7.5w fork oil by mixing equal parts 10w with 5w fork oil.? Just remember to relabel the container 7.5w.? It’s that simple.

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Todd Trumbore

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I came across an old BMW Tech Bulletin that specified a change in fork oil volume compared to original spec. Now says for Initial Fill 250ml (was 280) and for Oil Change 235ml (was 250). Change apparently made in response to harsh ride comments from riders/dealers. I'd be interested in community feedback on this volume issue since I'm getting ready to change the fork oil in my new-to-me '76.
Further it seems the original 7.5w spec fork oil is no longer available and 10w is now recommended.