Hi everyone. I've searched prior to posting with no luck, just confused. I have two older GM V8 powered engine cars. 1975 Pontiac GrandVille, 455 1980 Cadillac CDV. 368 I'm lost as to what spec oil I NEED. These cars sit in an unheated garage in NC, mild climate. I used the QS 10w-30 with added zinc levels in the 455 this spring. Is this appropriate? What change intervals would be recommended(I change in the spring yearly now)? Thanks for your time. SL.