regarding full synthetic oil

I have always considered 20/50 a VERY heavy oil for our engines. That said, I've seen a LOT of people run it through the years and every time without fail they had great service from it. I tried it in my slant 6 and it literally made it feel like it was bogging down trying to start cold. It would start and cut off several times. I could also tell a difference in how the engine ran. It would not idle well until it got warm. I ended up switching it to the same I run in everything now. Motorcraft 5/w20 synthetic blend and zinc additive added to it. As soon as I made that change, I felt an instant difference in how the engine ran. I couldn't believe it, but that's what happened.