My big six is picky about what brand of oil I run in it. If I run anything other than Pennzoil, it sounds like it is knocking. It doesn't matter what grade of oil it is, and there is no question about oil pressure (30 at idle, 50 at 2000rpm). Any ideas? Just curious. Thanks for any input.

 

Wow! That's really weird. Some have said their motor runs quieter with a thicker oil. I can't imagine why your big six only runs quiet on Penzoil. I wonder if the folks that frequent the "Oil and Lubrication" forum at this site would have any thoughts on that.