I've been using Valvoline in everything for 30+ years with no issues. When I bought my new Mazda 6 a few years ago, I figured, OK, high tech, time to try synthetic. Was surprised to have the service tech say the manufacturer recommends conventional oil!!!
But, I agree with a previous post, my '40 was not designed for anything but nasty 30 or 40 weight oil. And, since it has not been rebuilt, I'm sticking with it. HOWEVER... if I have it rebuilt, knowing everything is new and tight, I may re-consider synthetic.
