Blackstone sez:
Hi, Dan. We re-ran that viscosity test at 100°C (twice, just to be sure), and came back with a very similar reading of 15.913 cSt, or 81.6 SUS. This is slightly higher than the test we ran yesterday, but still not quite in the SAE 50-weight range.
A slightly lower viscosity isn't all that unusual with multi-weight oils, however. We base our "should be" values on the observed ranges of the oils we've tested, and we find that oils with a larger gap between the winter-weight and the SAE-weight will tend to have a slightly lower viscosity range than a multi-weight with a smaller difference between the two numbers, or a straight-weight oil of the same viscosity.
For example, a 0W/50, 20W/50, and straight SAE 50-weight oil should theoretically all have the same viscosity range at 100°C, but that's never been the case in any of the oils we've tested. The straight 50-weight oil should fall within the proper range for SAE 50 oil, but we expect the 20W/50 oil to read a bit lighter, and the 0W/50 oil to read even lighter still. We've tested samples from many different brands of 0W/50 oil, both virgin and used, and most fall at the light end of the 50-weight range or slightly lower.
Hope that helps. Let me know if there's anything else we can do for you. Thanks!