ATF... Synthetic, or not.
 

My car is an 83 300D with 220,000 miles and running like new. It has had regular transmission fluid changes, probably with whatever was cheapest. I intend on keeping it until the cows come home, and want to know what is the best ATF. Is it worth the slight expense to go synthetic or is regular stuff fine? Will their be any problems going to synthetic (keep in mind its had regular changes and stood up to the new fluid with no problems)

If synthetic is the best choice, give me your opinions on Mobil1, redline, and amsoil. Thanks. The engine currently has Mobil Delvac-1 5w-40 synthetic diesel oil and the rear end has valvoline synthetic 80w-90.

I switched to Mobil 1 synthetic ATF at 90,000 miles, mostly because my car sits outside during our usually very cold winters, and synthetic tends not to thicken as much in really cold weather.

I'm not sure you'll get much bang for the buck, unless your car is exposed to cold. You've obviously done a great job maintaining this vehicle to have 220 k on the transmission!

What is Seafoam additive?

Go with the Redline or Amsoil
 

Redline & Amsoil both make 100% synthetic fluids, and if you're going to switch to synthetic why not use the best.

Lots of stuff that I didn't know about oil:
http://www.peachparts.com/shopforum/tech-help/30429-pickup-truck-records-one-million-miles-pennzoil%AE-motor-oil.html

More if you want:
http://www.peachparts.com/shopforum/tech-help/31001-oil-additives.html