If my Check Engine light is occasionally coming on for a rich fuel mixture on the highway how do I adjust it so that stops happening?

Has the lean stop been messed with it? If it is too high, it will do that. I know a bit of a pain, to watch it in operation, get a dwell meter, connect it to the green lead on the engine by the carb, put on 6 cyl scale and drive with it with the meter next to you. It SHOULD stay in the 50% range if all is ok.

Basically it is saying the O2 sensor read >0.45v for more than a prescribed amount of time. Don't rush off to buy an O2 yet. But that could be a sign of one dying. But in my experience, most die lean not rich. AC Delco sensors are worth the $$. Trust me on that one.

Check the obvious, air filter, plugged exhaust, choke flap not fully open, bad ground on the exhaust for the O2 (it uses the exhaust for a ground path, not the best idea from GM...). Probably more I am not thinking of, I will ponder it later. What is the code, I have my 85 Olds FSM that covers it well.