I have used the Castrol Dextron-III ATF for many years and it is good stuff.

Just recently I had a slight "bitter-cold" shifting flair between 3rd and 4th gear, so I did a filter and fluid change (on the 1982 300D). This time I went with the Quaker State 4x4 Synthetic Blend ATF to see if it did better in the cold weather. It did.

So I'll probably continue using Castrol in everything but the 1982 300D, which will get some kind of synthetic.

It's almost like asking what motor oil to use - you're going to get a lot of suggestions because there are so many brands that are probably essentially the same in quality.

Ken300D