Hmmm....Is wear relevant in how a motor oil performs? Hold on....let me ask my cat.
OK. My cat just got back to me and said, "Meow. Yes, the ASTM D-4172 'Four Ball' test certainly does have relevance to engine wear. Meow." Gee, what a surprise.
I just skimmed through about a dozen oil producers' spec sheets, and unfortunately, none of them list this spec for reasons unknown, and I don't know the answer to the variations in the Mobil weights. If one's product does not result in good test numbers, better to not even mention that test in spec sheets -- that's one possibility.
As for what the Four Ball test is about, I've pasted this off two Amsoil websites: "The Four-Ball Wear Test is the industry's standard test method for measuring the wear preventive characteristics of a lubricant. Placed in a bath of the test lubricant, three fixed steel balls are put into contact with a fourth ball in rotating contact at preset test conditions. Lubricant wear protection properties are measured by comparing the average wear scars on the three fixed balls. The smaller the average wear scar, the lower the test number, and the better the protection."
"This machine has one ball rotating against three stationary balls under specific conditions of pressure, temperature, revolutions per minute and duration. At completion of each test, the wear scar size is measured on the stationary balls. For an example, a four-ball wear test of Amsoil synthetic Series 3000 5W30, Mobil 1 synthetic 5W40, Shell Rotella T 15W40 and Cummins premium blue 15W40 gave these scar sizes: Amsoil, 0.391mm; Mobil, 0.41mm; Shell,0.683mm; and Cummins,0.707mm.
Here's a helpful, short article on oil labs and ASTM D-4172:
http://www.bullittarchive.com/Maintenance/Oil/
[ October 21, 2003, 06:00 PM: Message edited by: TC ]