You are absolutely right about my mistake using the word index instead of number. Clumsy of me, especially because it leads to improper thinking of the subject, by both the reader, and myself.

If you don't mind, I'd like to further clarify (confuse?) two points that seem to need it;

1, When you compared SAE 90 gear oil at 210 degrees F and said it will flow like SAE 40 engine oil, both oils are at 210 degrees F.

2, When you referred to viscosity enhancers with regards to the SAE 40 weight motor oil so that it wouldn't pour like molasses when its cold, you were refering to making it a multi-weight oil. Like 10w40. There are no viscosity enhancers in straight weight oils. It's just the multi-weight oils that use them.

There is a down side to using viscosity enhancers. They make the oil weaker. The wider the span, the more easily damaged the oil molecures are. A straight 40 weight will hold up better to shear and heat then a 20w40. And a 20w40 will hold up better then a 5w40. The wider the span, the weaker the oil. I bring this up as a point only because multi-weight oils are common here. People do need to use care and judgement on how wide a span then need based on their climate conditions.

SAE has once again changed their web page, and now locked everything away, so I can't link to them directly. But the attached link has right at the top a good graph showing the relationship of gear oil and motor oils at 100 degrees C, which is what I was trying to describe. It also contains a heck of a lot of general oil information, from an undisclosed time in the past (it's dated, so use the information with care).

Everything you never wanted to know about oils.

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