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If you are referring to the 2020+ Defenders (L663), then get EXACTLY this for the first two years. After that, knock yourself out on anything that someone tells you would be a good substitute.
If you are referring to the pre-1984 Series III's, then I would ping these folks for your brake fluid needs: https://www.roverparts.com/land-rove...s/brake-fluid/. Personally, I'd stick with what was recommended as the price isn't a big deal. If it's what you have on hand, I'd get some of the correct type. Any local auto shop should carry it if you don't want to wait for it to be shipped.
Good luck. My Series IIa had drum brakes and it was TOUGH slowing down. Not that it got that fast to begin with! But, I'd try to help those old brakes out any way I could.