The Enviromental Protection Agency has agreed to allow Sears Roebuck & Co. Inc. to pay a substantially reduced cash penalty for violating unleaded gasoline regulations in return: for an agreement by Sears to warn customers of the enviromental perils of unleaded gasoline.

Sears agreed to mail a message to its approximately 25 million credit card customers extolling the virtues of unleaded gas in reducing air pollution and to decorate more than 16,000 Sears Vehicles with the same public service announcement. In return, instead of a $189,000 penalty, the company will pay only $20,000 and print the enviromental message on the bottom of its regular monthly statements.

"We beleive this agreement will promote public awareness of the air quality benefits of the use of unleaded gasoline as well as discourage future illegal use of leaded fuel," said Richard G. Kozlowski, the EPA's acting deputy assistant administrator for mobile source enforcement. The modified penalty was the EPA's idea.

"I'm always skeptical about things sent out with bills. I don't think people read them," said Paul Portney, a senior fellow with Resources for the Future. "Misfueling cars is a potentially serious business."

Sears is being penalized because employes at its service facility in Shreveport, La., filled the tanks of service vans and a company station wagon with leaded gasoline which the 1979-model vehicles required, according to the complaint. EPA issued the complaint in February, assessing a penalty of $189,000.

The vehicles were equipped with cataltyic converters, an emission-control device that can be rendered useless by leaded gasoline. Several tanks of leaded fuel can destroy a converter and cause emissions to increase eightfold, according to the EPA.