OK, OK, I'll jump in. I have two thoughts on the matter.

Slow, flaring shifts and slow engagement when shifting from park/neutral are typical symptoms of low fluid level. How certain are you that you're reading the fluid level correctly? It can be a bit tricky. Fluid from the dipstick tube (as opposed to the pan) gets on the dipstick and gives a false high reading. Since it can be difficult to see good, clean fluid on the dipstick, I use the following technique. Check the fluid when the light is good. Read both sides of the dipstick - front and back. Check it three times. When you get consistent readings front & back two times in a row - that's the true level.

Second thought. Some models have a gizmo which softens the shifting of the transmission until the engine reaches a certain temperature. I have seen transmissions immediately gain a cold flare when switched to synthetic fluid. The solution is to disconnect this "shift softener" device. It's as simple as unplugging the electrical connector from a switchover valve. Don't know whether your E420 has such a thing, but if so you might want to disconnect it. You'll have to search the electrical/vacuum diagrams for your car to find out if it exists. What brand of transmission fluid are you running?

- JimY