I just changed the oil in my 340: 4 quarts of 10-40, plus a quart of that Mystery Oil to see if it would help what sounds like a sticky lifter. While at it I put on some new valve covers. Cheap-ish Edelbrock covers with a breather in one and a PVC in the other, cork and rubber gaskets (I had old cop car 318 covers with rubber and steel gaskets on it before, also w a breather and PVC but 10 years old). After, my idle speed was way up, had to crank it down almost 3/4 turn of the idle screw. Is this because the oil is thinner? Or something about airflow with the new (tighter) valve covers?

vac leak-PCV issue.

I'm thinking something with the PCV, too.

Probably half PVC and half 'Mystery Oil". I used to know of a product called "Mix-I-Go" and you would add it to your fuel and listen to your idle go up and watch the black disappear from inside the exhaust pipe.

put the old pcv back in and see how it runs.

Ding ding ding I do believe we have a winna.