It took two years to convince myself to switch from the Honda Z-1 ATF to a synthetic ATF. Ended up going with Amsoil ATF which has been fantastic. Have also heard good things about the Redline D4 ATF. Biggest regret is not switching sooner!
My 2003 Ody with 170,000+ on the original tranny shifts much better with the Amsoil when compared to the Honda Z-1, seems to run smoother at initial startup in the winter, saw a slight increase in mpg and it seems quite clean when doing draining it.
The Honda Z-1 always smelled burnt despite changing it every 15,000 miles, the shifts felt rough and this was even worse during the winter months.
There are those who will swear by the Honda Z-1 and they are either changing it often or have bought into the "Honda Only" which you see printed in your owners manual on virtually every other line.