For your 1990 (and all models from '84-on), you're supposed to use Dexron. Dexron, Dexron-II, -III, etc., are all just successive generations based on new standards at the time -- use whatever is the current iteration.

By the way, for any readers with a pre-'84 car, you're supposed to use Type F instead of Dexron.

As for suggestions, I've used synthetic ATFs (as long as they're compatible with the above types) for about ten years, and won't use the regular stuff anymore. It's more expensive, but so is a transmission; and the synthetics last longer before they discolor (a sign of breaking down), so I'm convinced it's worth it. But that's me. At least use the right type.

Regards,