How many miles with graphite oil?

I put some graphite oil in my 1975 eight- cylinder Lincoln automobile; however, the oil was so black that I became leery of using it. Will the graphite harm an older engine? Does it really increase gasoline mileage by 25 percent, as advertised? When I need to add oil -- not an oil change -- can I use a can of regular, non-graphite oil? Vivian Winterman Jennings, Mo.

You're right when you say that Arco's graphite oil looks bad, but that doesn't mean a thing. It does its job in lubricating the engine, no matter what the car.

It's hard, however, to tell whether or not you will enjoy higher gas mileage; and if so, how much.

The Society of Automotive Engineers, the Department of Energy, and the petroleum industry are trying to come up with a system by which all competing lubricating oils can be measured in terms of higher gas mileage. In other words , right now you have only the word of the manufacturer, whether it be Atlantic-Richfield (Arco), Exxon, Mobil, or anyone else.

A major key to improved mileage on the road is the use of conservative driving habits; an in-tune, well-maintained car, etc.

Can you add a can of regular oil to the graphite oil? You ask.

By all means, but do not mix a synthetic lubricant, such as Mobil 2, with a petroleum-based oil.