That motor has a relatively high compression ratio.
It is one of the few motors (in the general automotive world) that actually benefits from higher octane fuel.
I generally run mid-grade with good results, but I bet my compression [at 215k] is not as good as yours is after that head rebuild. If the head was shaved, compression could be even higher than when new. I would bite the bullet and go for the premium.
1986 300E 5-Speed 240k mi.