What great cars! Bring back the big American iron and I will relive my youth.
My Mom always drove big Buick Electra's back in the 60's and 70's. My Dad drove Thunderbirds, but he did have a 1970 Olds Toronado that I liked. My first car was a 1966 GTO (Tri-Power, 4 speed) but when I had a hot date I drove my Mom's Buick (the back seat was huge!!)
Back on topic, if I had one of these cars today I'd run either a 15W-40 or 5W-40.