When one sees 10W40, or 80W90, the oil weight is actually the first number, but has additives (one example is STP) which makes the oil perform at the high temperature specification temperature preform the same as the larger second number. I think these numbers are 25C and 100C (been 45 years since engineering school, so....). For example; 10W40 has the 10 weight viscosity at the lower temperature, but 40 weight at 100C. With out the additive, the high temperature viscosity would be much lower.
Hope this helps