PH diphthongs

Probably yes.

Which ones in particular? *au, *āu, *eu, *ēu, *ou, *ōu?

Did Hellenic have long diphthongs?

I have no idea. This seems like a question for Sihler, but I'm not near my Sihler at the moment. We currently list *āi, *ēi, *ōi?

I wonder what PIE sources there are for these. As far as I know, Osthoff's law applied to Greek too.

Fair point. I suppose we can discount the hapax Ζηύς (Zēús) < *dyḗws as leveled from Ζήν (Zḗn)? I doesn't apply to diphthongs due to vowel hiatus like *néh₂us > *nā́us > νηῦς (nēûs), though.

Also, γραῦς (graûs) and maybe Θρᾷξ (Thrâix)

Sihler does not think there are long diphthongs and particularly explains away νηῦς. So should we remove them?