Your point is well taken, and I'm not trying to be confrontational please understand that. Again my beef is with the 40C and 100C numbers, % wise there's quite a difference. If in your carpenter example one carpenter was off as much % wise in his measurements there'd be a big problem, and he'd probably be looking for another job.. I work with them and I know how they measure, the difference could be in the tape measures used, or how one reads it, etc. Maybe I'm reading a little too much into this.
Originally Posted By: simple_gifts
How much faith is warranted? Very few people understand the testing methodology, the allowed variation etc. It is just a WAG that "these numbers aren't good enough"
What you say is true, however you have to have some faith in the data or it's worthless.
It is not about belief. Ask a few carpenters to cut something @ 5' 3 3/4" inches. It won't be 5' 3 3/4" but that doesn't matter as it will be "good enough" for what it is being used. I understand your concern. I'm not defending the variations wrt to viscosity; I'm suggesting (having worked in mfg) that "knowing" is a fleeting thing. Any measuring device needs to be 10 times more accurate than the precision to which you are trying to measure. If an oil fails into the 'light 30 weight' it might read 'high 20, light 30 even medium 30' it may make little difference if the oil reads 50 weight after 11K miles of service. If anything it may highlight the potential foolishness of choosing one oil over another based on .x variations in cst.
At this point I'd have no clue on which one to believe,