Idle Issue

As soon as I saw that picture I said "looks like Pennzoil to me" before I even saw what you wrote. :D
Yup. I used to have to have to read a lot of MIL specs and I found one on motor oils once; it prohibited the use of high parrafin content crudes, and that included a lot of the PA crudes. My 351C was that way.... the hood on the original Torino had Quaker State stickers all over the underside of the hood when I pulled it from the j-yard!

And KrazyKuda shows a good pics of where all that gunk goes.... my /6 pan looked just like that. If the OP does any internal cleaning, be ready to get the pan off to get that stuff out. Unless someone knows of a way to get it out via the oil drain hole....maybe flush with gasoline being veeeery careful AFTER removing the battery from the car....?? (Not a good procedure for smokers!)