My 04 pilot has the "D" flashing on the gear selector.

The 'D' light flashing is an indication that a DTC has been stored in the trans control module.Need to have it scanned with a scanner capable of communicating with TCM (not most generic code readers). Most likely will be a code for one of the pressure switches. Post code here and I can further advise. As for the trans fluid , if it's been more than 30K since it was serviced , I'd recommend a simple 'drain & fill , with HONDA ATF. Aftermarket may end up causing torque converter 'lock-up clutch' problems , from what I've seen. ---------How many miles are on it?