I predict that going to a lighter weight oil will improve the gas mileage from unspeakably lousy to unbelievably lousy. In layman's terms that amounts to about .2 of a mpg.
You can achieve far more mpg gain by backing off the rear brake shoe adjustment a bit so there is less drag, making sure the tires are at the highest pressure that you can run without inducing poor tire wear, making certain that the engine is tuned as best as possible, installing a vacuum gauge and drive by that, etc., than you'll ever get by running a thinner oil.
Does the 318 still have the Lean Burn set up on it ?
Back in the day the 318s were known for having pretty good gas mileage, compared to GM and Ford, and still had decent power. I'd bet there are a few mpgs still to be had with the engine tune.