I've actually owned that exact year and model Mercedes. Great road car and I liked it a lot. But I did like the engine in my 85 300SD a little better, ran Rotella 5w-40 HDEO in that. But for the 420SEL I'm going to cast another vote for neither HDEO. I owned mine when I lived in New Mexico and North Texas, in the 100's in the summer easy. Used multiple brands, Castrol, Mobil, MB oil, but all 0/5w-40 MB approved oils and it ran great. Wife got it in the divorce with over 140k miles on it.
As others have stated above, for euro vehicles it's the spec that matters. The HDEO would probably be "OK", but would not be the best choice. Any MB 229.5 oil will work great with your intended usage. In the end it's the whole package that matters, not just HTHS, Zinc, or Moly, etc. If you're hooked on Amsoil they happen to make a good 5w-40 FS Euro oil that has that approval and is decently priced for a Boutique oil. Otherwise a euro oil from Castrol, Mobil 1, Valvoline, etc will serve you well.