I have been using 5W30 in my 1950 Mercury and my Mark II for years. In many cases under average climate and driving heavy oils are not necessary. If you are mainly driving in city traffic or short trips heavy oil isn't a good idea I have seen over the years and often leads to sticking lifters or sludge as well. Lighter oils tend to flow better and keep the engine cleaner as well. GM Oldsmobile has had 5W30 recommended for years including the 70's and for those who think they need heavy oil (unless you are living in Nevada or Arizona?) look under the hood of Ford Pickups and get a load of the 5W20 on the filler cap.