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The oil usage could be a factor in the low mileage too. If the car is burning a quart every 300 miles the engine must be pretty worn. My '88 Escort with 490,000+ miles is only using a quart about every 1000 miles. I'd run a compression test just to see what the numbers are across the cylinders. The numbers between the cylinders shouldn't vary more than about 10-15% on an engine in good condition. If they come up low give them a squirt of oil into the cylinders from a pump oil can and recheck the numbers. If the numbers come up then your rings are worn out, if they don't then it's probably a head gasket or valve problem. If it's using that much oil you need to take a look at the plugs too, one or more of the plugs might be bad. There could be a bad plug wire. There are many things that could contribute to poor mileage. A bad 02 sensor will usually cut mileage by about 25-35%. Have you checked to see if there are any codes stored in the ECU? That's probably the best place to start.
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