years ago i had a 390 in a 66 fairlane, always used 20/50 as it was pretty 'loose'. i needed an emergency top up during a long run once and used 10/40 (all i could get) to get me home. that thing was noisier than hell. i changed it back to 20/50 the next day and it was all quiet as before. i know it was a ford but you guys are talking ketchup and molasses so.........
anyway maybe try the 15/50 as suggested.
neil.
