Oldsmobile officially unveiled the first American made diesel engine automobile in recent history, put a price of $740 in the Olds 98 as an option and $850 in the Olds-88, and predicted 1978 model sales of about 55.000.

R.J. Cook, general manager of the General Motors Corp. Div., said the V8 diesel tested at 21 miles per gallon in the city and 30 mpg on the high-way, yielding a composite mileage of 24. That vastly betters the 15 mpg city, 11 mpg highway and composite 17 mpg for the gasoline version of the 350 cubic inch engine.

"You're going to see a lot more diesels in the next two to three years," he said.STCook said he expects the diesel Oldsmobile to generate "a large amount of showroom traffic and enhance Oldsmobile's engineering reputation."

The diesel can't be sold or defended solely on grounds of saving money, he acknowledged. Over the life of the car a 7 per cent a gallon cheaper diesel fuel would not pay all of the option costs.

But the resale value should be higher, and in Europe used diesel cars recapture about three-fourths of the option costs, according to an Oldsmobile spokesman.

James Mattox Oldsmobile's general sales manager, said the more frequent oil change - every 3,000 miles compared with every 7,500 for the gasoline engine - roughly equals in cost the savings from having no spark plugs or ignition points to change.

Mattox said approximately 65,000 1977 Oldsmobiles were sold with Chevrolet engines yielding the "Chevymobile" flap of last winter and spring.

He said three-fourths of the owners elected the substantially enriched warranty they were offered. Another 15 per cent haven't been heard from yet, and about 11 per cent chose to take new cars.

"A lot took a different color, left off power windows, or took a different interior." Mattox said. Of those electing a new car," many accepted the Chevrolet engine in the exchange," he said.