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I Watched the Mileage Jump

I bought a 1997 Volvo 960 back in Feb 2005 with 66K miles on the clock. The previous owner supplied all the maintenance records with the car, so I knew its history. He changed the oil as recommended every 5,000 miles at the dealer, using mineral oil. Since the oil had been changed at 65K miles, I waited until 70K miles to change the oil and do an Auto-Rx treatment. The car was getting a solid 26.2-26.3 miles per gallon on the highway before the cleaning. I changed out the oil and filter and tossed in a bottle of Auto-Rx. While on a trip of 900 miles during the early part of the cleaning phase, I watched the mileage jump to 28.8 miles per gallon, then settle down as time went on to 26.8-27.0 mpg. On the second rinse phase oil change, I watched the mileage creep up to 27.2-27.3 mpg. The camshaft and valve train was coated with a dark chocolate brown varnish, borderline sludge coating at the beginning and after the first rinse phase the same area under the oil cap was as shiny as a brand new penny. One thing I did notice about 500 miles into the cleaning phase is that the engine-a 2.9-liter, 24-valve, inline 6-cylinder-did smooth out a lot at idle. At highway speed, I couldn’t really tell, but at idle I could definitely hear and feel the difference.
—A. Sichelstiel