Hi Frank,
I bought the car 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 it's 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 to 27.0 mpg. After the rinse phase was done and 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 border line sludge coating at the beginning and after the first rinse phase the same area under the oil cap was as shinny 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 defiantly hear and feel the difference.
Al Sichelstiel
ALS on BITOG.