It's never accurate and the ASTM test indicates it should be used only for screening purposes. Even then, flash point measurement has a low reproducibility compared to other property measurements and when another property is estimated off of this inherently imprecise value you get the problem with this method.Would you say that flashpoint is not an accurate indication of whether or not there's fuel contamination?
If VOA flashpoint is 455f and my sample reads 430f, can't be much fuel in there, me thinks.
Plus we have just seen various results from them to be inaccurate, not just flash point. If you want an accurate fuel dilution measurement you won't get it unless it is measured directly.
Mechanical shear of the VM is something that is blamed on here quite often when in reality there is no definitive determination that it exists. Without accurate fuel measurements one has no idea why there is a viscosity deviation. Fuel dilution is far more common a cause. One can eliminate VM mechanical shear if you use a no-VII oil.