I have not done much work on the S2000, so no specific knowledge of late stuff.

However a few years back I rebuilt the power steering rack on the mid 80s Prelude we had bought a daughter as her first car.

I was a bit doubtful of the requirement to use only Honda power steering fluid, until this time. When I found a teflon oil ring on the rack, I realised why the special oil requirement.

Some after market ATFs will be OK with teflon bits, but some will make teflon swell a little. This may not matter, but as Honda have tried to make the thing just a little better than the norm, I am prepared to use their recommendations to help keep the thing that way.

Sure some of those recommendations may be to keep you spending at the dealers, but some at least are best for your car, & worth following.

Pity my nearest Honda dealer is about an hour away, & won't deliver to private premises.