White cedar shingles
Cabot bleaching oil

They are actually Alaskan Yellow Cedar shingles with Cabot bleaching oil.

Do alaskan yellow cedar shingles weather to silver naturally...

Yes, they do and they don't have the tendency to "weather" black on northern exposures like red cedar tends to.

Is the bleaching oil necessary? Will they weather silver grey on all sides of house, and would you recommend yellow alaskan cedar side walls natural to weather over red western?

I do to like the black look, but I would like the shingle to weather naturally.

The bleaching oil helps "even out" the look on all elevations. If you don't mind some color/shading disparity between the elevations then the bleaching oil is not necessary. I would recommend the Alaskan yellow cedar over the red cedar if you prefer a more silvery gray uniform look.