That's bad info because it's BS.
Even though the SAE viscosity rating system is totally inappropriate for suspension fluids, ATF would be roughly a 10-15W oil.
The real problem with the SAE system, and suspension fluids, is that one companies 5W oil may be heavier than another companies 10W oil.
I did a lot of suspension tuning on my race bike with viscosity. If you have a suspension that you're not happy with, it's the second thing you should do after springs, and you should ALWAYS pursue it before even thinking about a revalve. If I used oils based on SAE viscosity, I would never be able to tune it.
Here's a chart that shows the viscosity's in centistokes. A much more reliable way of classifying suspension fluids.