Ok, one idea that I have also recently tried. The air cleaners on my trucks are all oil baths. I would run 50 wt or 30 wt which works fine one it is hot. But when cold it is to thick and cause the truck to run rough at idle. the syptom goes away the instant I accelerate. I have began to put in about 70 percent oil(I have lots of 30w), 30 percent marvel mystery oil. the problem has gone away. Try it for grins and giggles. If you are running a paper element try the coil. I had the coil go bad on my 49 216 while dirving it to yellowstone. Caused sporadic issues, including rough idle. But my bet is the air cleaner.
