My 3550 shifted just a tad better with BG Synchromesh II, the
synthetic fluid, had used Redline D4, and MTL.
I do think you should drain it hot, that way you get out all the tiny microscopic brass synchro particles, i just stick a plastic tube into the fill hole, run it up side the passengers side of the motor, put a funnel on it, and slowly pour in warm trans fluid, mine takes just a tad under 3 quarts. When it runs out, alongside the fillhose, just pull it out, and put the fill plug back in.