I'm making a blind guess here:
I never tried to edit 3D Objects in OMSI, but since the anim_rot of the MI8 is governed by Rotor variable, maybe trying to change that 500 to a lower or greater coefficient should solve that?
(L.L.Rotor) (L.S.Timegap) 500 * + (S.L.Rotor)
(L.L.Rotor) 360 - (S.L.Rotor)
Maybe lowering the value may cause the rotation to slow down.