First, I know Honda doesn’t make the fluid. Second, I wouldn’t take Castrol’s word for it. Third, you can buy Honda OEM products online as cheaply as you can buy Castrol’s product from a store, so you aren’t saving any money. You don’t have to pay inflated prices by buying OEM Honda products at the dealership.
For all we know, Castrol might be the maker of Honda’s Z-1 fluid. If that is the case, I still wouldn’t assume the products are made to the same specifications.
Knowing all this, why would you take the risk? What do you expect to gain? You won’t save money and the Castrol product might not be as good as advertised.
I will ask again, what does the owner’s manual say? If the owner’s manual lists the specifications the fluid must meet, and this product meets those specifications, go for it. If you are just going by Castrol’s claims, good luck. Those claims might be a rare case of truth in advertising, but I wouldn’t bet on it with nothing to gain.
Jeffmw05, do an internet search for “Honda OEM parts.” The last time I ordered OEM parts online, I got them for much less than I would have paid at the dealership.