Thats condescending to claim I am "furthering ignorance". There is no need take a shot like that. I am not surprised though, since you have 60K posts here, and you very much like to debate. I get that. This subject has come up before. Its not germane just to this thread or poster. No one is claiming 12.5 is technically valid number, but it certainly does give the gist where the viscosity lands, at least to those who can understand the nuance of it. Maybe you just can't.This example is like measuring battery capacity in "miles" instead of kWh, it might seem "easier" but it's ultimately nonsense because it's trying to apply a static measure to a variable or range. You think you are helping by making it "simple" but ultimately, by avoiding the proper units, you are just furthering ignorance because the person doesn't leave the conversation with any greater understanding of the material than when they arrived.
There is no "seat of the pants number" because grades are clearly defined ranges in SAE J300, which has been posted countless times on this forum and should be the basic starting point for anybody interested in this subject.
Clearly, the OP appreciated a more detailed and accurate answer, given his response to my reply.
Technical precision has its place. I have a 40 year career of technical precision with analytical chemistry in a QA/QC laboratory. I have great experience with gas and liquid chromotography, and GCMS. I work with ppm and ppb analysis daily. I don't need a condecending lecture on technicalities. I understand precision and technical accuracy more than most anyone but a high degree of such is not necessary to convey a simple idea. Some people miss the forest for the trees. I do not envy them.