Also, keep in mind that every UOA has some fuel in it. Fuel will decrease the viscosity more at 40C than it will at 100C, so fuel added to oil increases the viscosity index of the mixture. The effect is very significant. Check it out quantitatively using Widman's site's mixing viscosities page. Also remember that Blackstone's fuel percentages are underestimated, so don't assume 0.5% or less fuel content. Over 1% is a more reasonable set of numbers to assume.