I know they are not directly petro, but I just listed four different auctions for Buick Dealer Postcards.

A 1954 Skylark Convertible Item#
190077489339

A 1956 Super Convertible & Super Riviera 2dr ht Item# 190077487124

A 1956 Century Estate Wagon & Century 4dr ht Riviera Item# 190077484343

A 1956 Special Lot of 4 Models Item# 190077448459

I have many, many other Buick Postcards from 1949-56 if anyone is interested please email me at andrew@innernet.net and we'll talk! Thanks for looking!