Yes, I would like to hear from the high timers, over 40,000 miles.
I think it all shears down to 30 weight after about 3k.
True, this has been proven in tests, actually most 10W40 will shear down to a 30 weight equivalent within 1500 miles. Typically synthetics take another 1000 miles. This is in a wet clutch motorcycle application with shared transmission, notoriously hard on engine oil.
I have argued for a long time that this is not significant. A lubrication engineer stated awhile back that most liquid cooled engines perform best with a 30W oil when at operating temperature. The reason the manufacturers don't specify a 10W30 from the start is because it will shear down to a 20 weight over time which is a bit too thin. By specifying a 10W40 like most do, it starts out a little thicker than optinmum then shears down to a equivalent 10W30 in the first 1500 to 2000 miles and then stays there. The majority of the viscosity loss (steep part of the curve) is literally in the first 1500 miles then it stabilizes almost indefinetly. Some oils will actually thicken up if you run them long enough due to soot accumulation.
A test done by a university physics professor showed that the original Mobil 1 synthetic lost 17% of it's viscosity in the first 1500 miles in a motorcycle application. Castrol GTX lost 32% and Honda HP4 lost about 34% of it's initial vsicosity in the same time frame.I'll Google it and see if I can find that test.
* Last updated by: Kruz on 9/22/2010 @ 11:43 AM *
2021 Aprilia RSV4
2020 BMW S1000RR
2016 ZX-10R KRT
2016 959 Panigale Red
2015 CBR1000RR SP Repsol
2011 ZX-10R Ebony
2009 ZX-6R Lime Green
2006 ZX-14 Red
2004 VTX 1300C Candy Red
"For we walk by faith and not by sight" II Corinthians 5:7