What oil are you using
We had started out with Royal Purple in our '70 Roadrunner but began to hear bad things about it. We were told that it would get dark and sludgy quick. Sure enough, after a few thousand miles that's what we saw too. We've still got 2 cases of it in the garage and will use it up but we probably won't run it between changes more than 2000 miles. Once it's gone we won't be buying it again.